I have read in the history of the church, that the early church believed that if a person sins, their interpretation was that since there is no more sacrifice for sin, they have lost their salvation, permanently. Under Roman rule, thru various caesars, some were Christian friendly, and some were violently opposed to Christianity, sentencing Christians to death. After a Christian friendly ruler, many became 'christians', but later, a very anti-christian ruler came to power and persecuted those who claimed to be christians. Many christians fell away, renouncing Jesus, returning to the older Roman religions. When the next christian friendly ruler came to power, some who had fallen away, bemoaned their sin, but the church taught that they had sinned and permanently lost their salvation. It was during this time that church leadership discussed the doctrines and decided that a person could repent, and be saved again.
But my point is, at any given time, we know that there are those who have 'snuck in unawares'. People who are not truly born again believers have joined various churches. When a time comes with persecution, and it does not have to be the great tribulation, there will be those who were among us, but are not part of us, who will 'fall away'. I view it as a very sad gleaning of the church.
This is affirmed further by the parable of the seeds.
Matthew 13: 18“Hear then the parable of the sower. 19 When anyone hears the [i]word of the kingdom and does not understand it, the evil one comes and snatches away what has been sown in his heart. This is the one on whom seed was sown beside the road. 20 The one on whom seed was sown on the rocky places, this is the man who hears the word and immediately receives it with joy; 21 yet he has no firm root in himself, but is only temporary, and when affliction or persecution arises because of the [j]word, immediately he [k]falls away. 22 And the one on whom seed was sown among the thorns, this is the man who hears the word, and the worry of the [l]world and the deceitfulness of wealth choke the word, and it becomes unfruitful. 23 And the one on whom seed was sown on the good soil, this is the man who hears the word and understands it; who indeed bears fruit and brings forth, some a hundredfold, some sixty, and some thirty.”
24 Jesus presented another parable to them, saying, “The kingdom of heaven may be compared to a man who sowed good seed in his field. 25 But while his men were sleeping, his enemy came and sowed tares among the wheat, and went away. 26 But when the wheat sprouted and bore grain, then the tares became evident also. 27 The slaves of the landowner came and said to him, ‘Sir, did you not sow good seed in your field? How then does it have tares?’ 28 And he said to them, ‘An enemy has done this!’ The slaves *said to him, ‘Do you want us, then, to go and gather them up?’ 29 But he *said, ‘No; for while you are gathering up the tares, you may uproot the wheat with them. 30 Allow both to grow together until the harvest; and in the time of the harvest I will say to the reapers, “First gather up the tares and bind them in bundles to burn them up; but gather the wheat into my barn.”’”
So, anytime there is persecution, some will fall away. The following verse concerns those who fall away due to false doctrines.
1 Tim 4:1 But the Spirit explicitly says that in later times some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons, 2 by means of the hypocrisy of liars seared in their own conscience as with a branding iron, 3 men who forbid marriage and advocate abstaining from foods which God has created to be gratefully shared in by those who believe and know the truth.
I personally believe that God promises to keep those who are His. There are many verses which indicate that we are safely in His hands, and He will not lost us, forsake us, and if one of the sheep wander away, He will come to get us.
From my own experience, I was not raised in Christianity, and when I found Jesus was Messiah and Lord, my family and friends did not receive the news well at all. My family declared me dead to them, my friends abandoned me, and my husband divorced me. To regain family and spouse, all I had to do was renounce Jesus. I also lost my job as my boss was of the same ethniciy and belief as my family. I know this sound all horrible, and it was not fun, but God gave me extraordinary faith, and peace thru that time. If Christians do go thru tribulation, God says He will not give us more then we can bear, and is capable of sustaining us. Since I was a new believer when this happened, I was not grounded in doctrine, and had not formed any end time belief. Yet, God was there. I had no real time to prepare for persecution, yet God was there for me.
I would not look forward to the great tribulation, but past experience tells me that whatever we have to experience, if we are genuine believers, God will be there.