This is a rather long explanation of what you're inquiring to. It's the explanation I subscribe to and makes biblical sense to me. It's very interesting and a worthwhile read.
Behold I will send (I send, as a future, proximate in the prophet's mind) you Elijah the prophet - The Archangel Gabriel interprets this for us, to include the sending of John the Immerser. For he not only says Lu 1:17. that he shall "go before" the Lord "in the spirit and power of Elias," but describes his mission in the characteristic words of Malachi, "to turn the hearts of the fathers to the children:" and those other words also, "and the disobedient to the wisdom of the just," perhaps represent the sequel in Malachi, "and the hearts of the children to the fathers;" for their hearts could only be so turned by conversion to God, whom the fathers, patriarchs and prophets, knew, loved and served; and whom they served in name only. John the Immerser, in denying that he was Elias, Joh 1:21 denied only, that he was that great prophet himself. Our Lord, in saying Mt 11:14, "This is Elias, which was for to come Mt 17:12 that Elias is come already and they knew him not, but have done unto him whatsoever they listed," met the error of the scribes, that He could not be the Christ, because Elias was not yet come. When He says Mt 17:11, "Elias truly shall first come and restore all things," He implies a coming of Elijah, other than that of John the Immerser, since he was already martyred, and all things were not yet restored. This must also be the fullest fulfillment. "For the great and terrible Day of the Lord" is the Day of Judgment, of which all earthly judgments, however desolating, (as the destruction of Jerusalem) are but shadows and earnests. Before our Lord's coming all things looked on to His first coming, and, since that coming, all looks on to the second, which is the completion of the first and of all things in time.
Our Lord's words, "Elias truly shall first come and restore all things," seem to me to leave no question, that, as John the Immerser came, in the spirit and power of Elias, before His first coming, so, before the second coming, Elijah should come in person, as Jews and Christians have alike expected. This has been the Christian expectation from the first. Justin Martyr asked his opponent "Shall we not conceive that the Word of God has proclaimed Elias to be the forerunner of the great and terrible day of His second Coming?" "Certainly," was Trypho's reply. Justin continues, "Our Lord Himself taught us in His own teaching that this very thing shall be, when the said that 'Elias also shall come;' and we know that this shall be fulfilled, when He is about to come from heaven in glory." Tertullian says "Elias is to come again, not after a departure from life, but after a translation; not to be restored to the body, from which he was never taken; but to be restored to the world, from which he was translated; not by way of restoration to life, but for the completion of prophecy; one and the same in name and in person." "Enoch and Elias were translated, and their death is not recorded, as being deferred; but they are reserved as to die, that they may vanquish Antichrist by their blood."
And, in proof that the end was not yet, "No one has yet received Elias; no one has yet fled from Antichrist." And the ancient author of the verses against Marcion;, "Elias who has not yet tasted the debt of death, because he is again to come into the world." Origen says simply in one place, that the Saviour answered the question as to the objection of the Scribes, "not annulling what had been handed down concerning Elias, but affirming that there was another coming of Elias before Christ, unknown to the scribes, according to which, not knowing him, and, being in a manner, accomplices in his being cast into prison by Herod and slain by him, they had done to him what they listed." Hippolytus has, "As two Comings of our Lord and Saviour were indicated by the Scriptures, the first in the flesh, in dishonor, that He might be set at naught - the second in glory, when He shall come from heaven with the heavenly host and the glory of the Father - so two forerunners were pointed out, the first, John, the son of Zacharias, and again - since He is manifested as Judge at the end of the world, His forerunners must first appear, as He says through Malachi, 'I will send to you Elias the Tishbite before the great and terrible day of the Lord shall come. '"
Hilary, "The Apostles inquire in anxiety about the times of Elias. To whom He answereth, that "Elias will come and restore all things," that is, will recall to the knowledge of God, what he shall find of Israel; but he signifies that John came "in the spirit and power of Elias," to whom they had shown all severe and harsh dealings, that, foreannouncing the Coming of the Lord, he might be a forerunner of the Passion also by an example of wrong and harass." "We understand that those same prophets (Moses and Elias) will come before His Coming, who, the Apocalypse of John says, will be slain by Antichrist, although there are various opinions of very many, as to Enoch or Jeremiah, that one of them is to die, as Elias."
Hilary the Deacon, 355 a.d., has on the words, "I suppose God hath set forth us the Apostles last;" "He therefore applies these to his own person, because he was always in distress, suffering, beyond the rest, persecutions and distresses, as Enoch and Elias will suffer, who will be Apostles at the last time. For they have to be sent before Christ, to make ready the people of God, and fortify all the Churches to resist Antichrist, of whom the Apocalypse attests, that they will suffer persecutions and be slain." "When the faithless shall be secure of the kingdom of the devil, the saints, i. e., Enoch and Elias being slain, rejoicing in the victory, and 'sending gifts, one to another' as the Apocalypse says Re 11:10 sudden destruction shall come upon them. For Christ at His Coming, shall destroy them all." Gregory of Nyssa quotes the prophecy under the heading, that "before the second Coming of our Lord, Elias should come."
Ambrose, "Because the Lord was to come down from heaven, and to ascend to heaven, He raised Elias to heaven, to bring him back to the earth at the time He should please." "The beast, Antichrist, ascends from the abyss to fight against Elias and Enoch and John, who are restored to the earth for the testimony to the Lord Jesus, as we read in the Apocalypse of John."
Jerome gives here the mystical meaning; "God will send, in Elias (which is interpreted 'My God' and wire is of the town Thisbe, which signifies 'conversion' or 'penitence') the whole choir of the prophets, "to convert the heart of the fathers to the sons," namely, Abraham and Isaac and Jacob and all the patriarchs, that their posterity may believe in the Lord the Saviour, in whom themselves believed: 'for Abraham saw the day of the Lord and was glad.'" Here, he speaks of the "coming of Elias before their anointed," as a supposition of Jews and Judaizing heretics. But in commenting on our Lord's words in Matthew, he adheres twice to the literal meaning. On Mt 11:14-15, "Some think that John is therefore called Elias, because, as, according to Malachi, at the second coming of the Saviour. On Mt 17:11-12, Elias will precede and announce the Judge to come, so did John at His first coming, and each is a messenger, of the first or second coming of the Lord:" and again concisely, On Mt 17:11-12, "He who is to come in the second Coining of the Saviour in the actual body, now comes through John in spirit and power;" and he speaks of Enoch and Elias as "the two witnesses in the Revelation, since, according to the Apocalypse of John, Enoch and Elias are spoken of, as having to die."
Chrysostom, "When He saith that Elias "cometh and shall restore all things," He means Elias himself, and the conversion of the Jews, which shall then be; but when He saith, "which was to come," He calls John, Elias, according to the manner of his ministry."
In Augustine's time it was the universal belief., "When he (Malachi) had admonished them to remember the law of Moses, because he foresaw, that they would for a long time not receive it spiritually, as it ought, he added immediately; "And I will send you Elias the Thisbite" etc. That when, through this Elias, the great and wonderful prophet, at the last time before the judgment, the law shall have been expounded to them, the Jews shall believe in the true Christ, i. e., in our Christ, is everywhere in the mouths and hearts of the faithful. For not without reason is it hoped, that he shall come before the Coming of the Saviour, as Judge, because not without reason is it believed that he still lives. For he was carried in a chariot of fire from things below; which Scripture most evidently attests. When he shall come then, by expounding the law spiritually, which the Jews now understand carnally, he shall turn the heart of the fathers to the children."
Cyril of Alexandria, his antagonist "Theodoret, and Theodore" of Mopsuestia, who was loose from all tradition, had the same clear belief. Cyril; "It is demonstrative of the gentleness and long-suffering of God, that Elias also the Tishbite shall shine upon us, to foreannounce when the Judge shall come to those in the whole world. For the Son shall come down, as Judge, in the glory of the Father, attended by the angels, and shall 'sit on the throne of His glory, judging the world in righteousness, and shall reward every man according to his works.' But since we are in many sins, well is it for us, that the divine prophet goes before Him, bringing all those on earth to one mind; that all, being brought to the unity through the faith, and ceasing from evil intents, may fulfill that which is good, and so be saved when the Judge cometh down. The blessed John the Baptist came before Him "in the spirit and power of Elias." But, as he preached saying, 'Prepare ye the way of the Lord, make His paths straight,' so also the divine Elias proclaims His then being near and all-but-present, that He may 'judge the world in righteousness.'" Theodoret;, "Malachi teaches us how, when Antichrist shall presume on these things, the great Elias shall appear, preaching to the Jews the coming of Christ: and he shall convert many, for this is the meaning of, "he shall turn the heart of the fathers to the children," i. e., the Jews (for these he calls fathers, as being older in knowledge) to those who believed from the Gentiles. They who shall believe through the preaching of the great Elias, and shall join themselves to the Gentiles who seized the salvation sent to them, shall become one church.
He hints, how when these things are done by Antichrist, Michael the Archangel will set all in motion, that Elias should come and foreannounce the coming of the Lord that the then Jews may obtain salvation." And on this place, "Knowing well, that they would neither obey the law, nor receive Him when He came, but would deliver Him to be crucified, He promises them, in His unspeakable love for man, that He will again send Elias as a herald of salvation, 'Lo, I will send you Elias the Tishbite.' And signifying the time, He added, 'Before the great and terrible day of the Lord shall come:' He named the Day of His Second Coming. But He teaches us, what the great Elias shall do, when he comes, 'Who shall bring back the heart of the father to the son' etc. And pointing out the end, for which Elias should first come, 'Lest I come and smite the earth utterly.' For lest, finding you all in unbelief, I send you all to that endless punishment, Elias will first come, and will persuade you, O Jews, to unite you indissolubly with those, who from the Gentiles believe in Me, and to be united to My one Church."
Theodore of Mopsuestia paraphrases: "In addition to all which I have said, I give you this last commandment, to remember My law, which I gave to all Israel through Moses, plainly declaring what they ought to do in each thing, and as the first token of obedience, to receive the Lord Christ when He cometh, appearing for the salvation of all men: Who will end the law, but show His own perfection. It had been well, had you immediately believed Him when He came, and known Him, as He whom Moses and all the prophets signified, Who should put an end to the law, and reveal the common salvation of all men, so that it should be manifest to all, that this is the sum and chief good of the whole dispensation of the law, to bring all men to the Lord Christ, Who, for those great goods, should be manifested in His own time. But since, when He manifested Himself, ye manifested your own ungainliness, the blessed Elias shall be sent to you before the second Coming of Christ, when He will come from heaven, to unite those who, for religion, are separated from each other, and, through the knowledge of religion, to bring the fathers to one-mindedness with the children, and in a word, to bring all men to one and the same harmony, when those, then found in ungodliness, shall receive from him the knowledge of the truth in the communion with the godly thence ensuing."
"The African author of the work on the promises and predictions of God." (between 450 and 455 a.d.)
, "Against Antichrist shall be sent two witnesses, the prophets Enoch and Elijah, against whom shall arise three false prophets of Antichrist."
Isidore of Seville 595 a.d.;, "Elias, borne in a chariot of fire, ascended to heaven, to come according to the prophet Malachi at the end of the world, and to precede Christ, to announce His last coming, with great deeds and wondrous signs, so that, on earth too, Antichrist will war against him, be against him, or him who is to come with him, and will slay them; their bodies also will lie unburied in the streets. Then, raised by the Lord, they will smite the kingdom of Antichrist with a great blow. After this, the Lord will come, and will slay Antichrist with the word of His mouth, and those who worshiped him.", "This will be in the last times, when, on the preaching of Elias, Judah will be converted to Christ."
To add one more, for his great gifts, Gregory the Great., "It is promised, that when Elias shall come, he shall bring back the hearts of the sons to their fathers, that the doctrine of the old, which is now taken from the hearts of the Jews, may, in the mercy of God, return, when the sons shall begin to understand of the Lord God, what the fathers taught.", "Although Elias is related to have been carried to heaven, he deferred, he did not escape, death. For it is said of him by the mouth of the Truth Himself, 'Elias shall come and restore all things.' He shall come to 'restore all things;' for to this end is he restored to this world, that he may both fulfill the office of preaching, and pay the debt of the flesh.", "The holy Church, although it now loses many through the shock of temptation, yet, at the end of the world, it receives its own double, when, having received the Gentiles to the full, all Judaea too, which shall then be, agrees to hasten to its faith. For hence it is written, "Until the fullness of the Gentiles shall come, and so all Israel shall be saved."
Hence, in the Gospel the Truth says, "Elias shall come and shall restore all things." For now the Church has lost the Israelites, whom it could not convert by preaching; but then, at the preaching of Elias, while it collects all which it shall find, it receives in a manner more fully what it has lost.", "John is spoken of as to come in the spirit and power of Elias, because, as Elias shall precede the second Coming of the Lord, so John preceded His first. For as Elias will come, as precursor of the Judge, so John was made the precursor of the Redeemer. John then was Elias in spirit; he was not Elias in person. What then the Lord owned as to spirit, that John denies as to the person."
Whether Elijah is one of the two witnesses spoken of in the Apocalypse, is obviously a distinct question. Of commentators on the Apocalypse, Arethas remarks that as to Elijah, there is clear testimony from Holy Scripture, this of Malachi; but that, with regard to Enoch, we have only the fact of his being freed from death by translation, and the tradition of the Church. John Damascene fixed the belief in the Eastern Church. In the West, Bede e. g., who speaks of the belief that the two witnesses were Elijah and Enoch, as what was said by "some doctors," takes our Lord's declaration, that Elijah shall return, in its simple meaning. (on Mt 17:11; Mark 9.) Yet it was no matter of faith. When the belief as to a personal Antichrist was changed by Luther and Calvin, the belief of a personal forerunner of Christ gave way also.