If you don't repent of willful sin, are you automatically forgiven?
That's a different subject. And you are correct, that repentance is the doorway to forgiveness.
What I am talking about, is the fact that Jesus was talking to the Pharisees, who were supposed to be keeping the Law. In fact, Jesus first referred them to the Law, when they asked him of the matter.
Mark 10:2 And the Pharisees came to him, and asked him, Is it lawful for a man to put away his wife? tempting him.
Mark 10:3 And he answered and said unto them, What did Moses command you?
Jesus would not have referred the Pharisees to the Law, and then commanded them to break it.
If Jesus had been telling the Pharisees that Moses' law in Deuteronomy 24:1-4 no longer applied, He would never have asked them "what did Moses say?"
What Jesus actually said here about divorce-in-order-to-remarry, being adultery, is actually found in the OT.
Malachi 2:14 Yet ye say, Wherefore? Because the LORD hath been witness between thee and the wife of thy youth, against whom thou hast dealt treacherously: yet is she thy companion, and the wife of thy covenant.
15 And did not he make one? Yet had he the residue of the spirit.
(God doesn't marry people on a spiritual level. He makes them one flesh, not one spirit.
He reserves the spirit for Himself, to be joined as one to himself --1 Cor 6:17.)
And wherefore one? That he might seek a godly seed. Therefore take heed to your spirit, and let none deal treacherously against the wife of his youth.
16 For the LORD, the God of Israel, saith that he hateth putting away: for one covereth violence with his garment, saith the LORD of hosts: therefore take heed to your spirit, that ye deal not treacherously.
If Jesus had been saying that all remarriage after divorce is adultery, then he would have been destroying the Law.
If He had been saying that all divorced couples need to break up their new homes, and go back to their former spouses, then Jesus would have been turning the abomination of Deuteronomy 24:4 into a commandment.
But the New Testament says the Law is holy, and the commandment holy, and just, and good.
Even Deuteronomy 24:1-4.
Yes, sin is perpetual, until it is repented of.
However, God does recognize marriages made in sin.
If it is true that all sinfully-established marriages need to break up, then where is the command for that in any of the Epistles? It isn't there. With all the problems Paul dealt with, in the Gentile church, surely this would have been a problem, especially in light of the fact that divorce and remarriage was so rampant in EVERY culture of that day!
But Paul spoke not one word of it.
And there is NO record of it, anywhere in the New Testament, being required of believers.