First - I’ll address why Jesus said this to the leper that he healed. The Torah has a very specific subset of Levitical rules around how to verify that a leper is healed. This is the first case of a leper healed after God gave the law to Moses, in fact the only known point in history this Levitical Law was ever used after it was written. It was so the Jewish leaders could see that a fulfillment of a messianic prophecy occurred. This is one example of where the Torah points to the messiah, and the Ancient Rabbis identified this is one of the three messianic miracles that is required to identify the messiah (the other two included Excorcism of a Mute Demon, and healing of a man Born blind).
To address the other points. The NT is full of examples & commandments where it shows a clear dichotomy between believers, and followers. “What good is it, dear brothers and sisters, if you say you have faith but don’t show it by your actions? Can that kind of faith save anyone? Suppose you see a brother or sister who has no food or clothing, and you say, “Good-bye and have a good day; stay warm and eat well”—but then you don’t give that person any food or clothing. What good does that do? So you see, faith by itself isn’t enough. Unless it produces good deeds, it is dead and useless. Now someone may argue, “Some people have faith; others have good deeds.” But I say, “How can you show me your faith if you don’t have good deeds? I will show you my faith by my good deeds.” You say you have faith, for you believe that there is one God. Good for you! Even the demons believe this, and they tremble in terror. How foolish! Can’t you see that faith without good deeds is useless? Don’t you remember that our ancestor Abraham was shown to be right with God by his actions when he offered his son Isaac on the altar? You see, his faith and his actions worked together. His actions made his faith complete. And so it happened just as the Scriptures say: “Abraham believed God, and God counted him as righteous because of his faith.” He was even called the friend of God. So you see, we are shown to be right with God by what we do, not by faith alone. Rahab the prostitute is another example. She was shown to be right with God by her actions when she hid those messengers and sent them safely away by a different road. Just as the body is dead without breath, so also faith is dead without good works.”
James 2:14-26 NLT
I think a few of Paul’s writings are often taken out of context (for example Romans 6:14) misunderstood and hyperfocused on to negate much of Jesus’ teaching. But yet if you read this verse in context, as well as the rest of Paul’s writings, you can see Paul clearly explains we need the law. “The Law” = The Torah. How can we obey God, as Jesus commands, unless we know what God wants us to do? Yes you may say that He writes his commandments on our heart, but where does this come from? God is unchanging. His law doesn’t change. Are the 10 commandments now irrelevant? Jesus sacrifice on the cross replaced the Levitical sacrificial system, which had a clear purpose for atonement of sin, but was an imperfect system replaced by our perfect lamb of God. The sacrificial system given to Israel by God pointed to the messiah, showing the cost of sin and showing that He would be the ultimate sacrifice. On the same Passover when Jesus died on the cross, 2000 lambs per minute were slaughtered to atone for sin, creating a river of blood from God’s Holy Temple! In regard to the rabbinical law, which is an extra-biblical set of rules, much of what Jesus Spoke against was the obsessiveness of the teachers of the law requiring people to obey the minutia, the detailed portions of the rabbinical law, while ignoring the big issues that are the fundamental principles of God’s laws - “justice, mercy, faith”. See Mt23:23 Tithing on the spices, not eating milk and meat together, obsessive hand washing... a list of rules to accompany the Torah - this is what Jesus spoke against only because the people replaced God’s law with these rabbinical laws.
Now regarding the Torah and it’s applicability to Gentile believers - Should we ignore the foundation of the tree that we, as believers, have been grafted into (Israel)?
Jesus’ life and ministry was very clear - His message of sacrifice and redemption was very clearly intertwined with a message requiring absolute obedience to His commandments. Jesus sums up the consequences perfectly in Matthew 7:21-23
““Not everyone who calls out to me, ‘Lord! Lord!’ will enter the Kingdom of Heaven. Only those who actually do the will of my Father in heaven will enter. On judgment day many will say to me, ‘Lord! Lord! We prophesied in your name and cast out demons in your name and performed many miracles in your name.’ But I will reply, ‘I never knew you. Get away from me, you who break God’s laws.’”
Matthew 7:21-23 NLT
In these verses, The people being condemned are clearly those who call out to Jesus, profess to accept His message, but break God’s laws.
If all we needed to do is believe in Jesus, then why would Jesus say:
“But the gateway to life is very narrow and the road is difficult, and only a few ever find it.”
Matthew 7:14 NLT
The truth is, much of Jesus’ teaching is hard. It was hard teaching then, it is still hard teaching today. It is not the fluffy message that our itching ears want to hear.
Most importantly, we need to hear Jesus’ words directly about the Torah:
““Do not think that I came to abolish the Torah or the Prophets! I did not come to abolish, but to fulfill. Amen, I tell you, until heaven and earth pass away, not the smallest letter or serif shall ever pass away from the Torah until all things come to pass. Therefore, whoever breaks one of the least of these commandments, and teaches others the same, shall be called least in the kingdom of heaven. But whoever keeps and teaches them, this one shall be called great in the kingdom of heaven. For I tell you that unless your righteousness exceeds that of the Pharisees and Torah scholars, you shall never enter the kingdom of heaven!”
for if we want to receive the Holy Spirit, which is how we receive God’s law on our hearts, we must first believe, Love Jesus and live in obedience to His commandments as per John 14:15-16 and all of John 15 - remain in Him and He shall remain in us.
“When you obey my commandments, you remain in my love, just as I obey my Father’s commandments and remain in his love.”
Anyway - this is what I am compelled to say. Blessings and Shalom.