Jeremiah prophesied to the Jews of Judea shortly before and during the time of Nebuchadnezzar’s attacks on Judea.
(Jer 4:4 ESV) 4 “Circumcise yourselves to the LORD; remove the foreskin of your hearts, O men of Judah and inhabitants of Jerusalem; lest my wrath go forth like fire, and burn with none to quench it, because of the evil of your deeds.”
(Jer 9:25-26 ESV) “Behold, the days are coming, declares the LORD, when I will punish all those who are circumcised merely in the flesh — 26 Egypt, Judah, Edom, the sons of Ammon, Moab, and all who dwell in the desert who cut the corners of their hair, for all these nations are uncircumcised, and all the house of Israel are uncircumcised in heart.”
This is an obvious warning, based on Deu 10, that the mere form of obedience is not good enough. God insists on hearts that love him. Contrary to over a century of really bad teaching, the Law of Moses was just as much about the heart as God’s other covenants.
Ezekiel prophesied in Babylon to the exiles there, often speaking of the end of the Exile —
(Eze 11:19-20 ESV) 19 And I will give them one heart, and a new spirit I will put within them. I will remove the heart of stone from their flesh and give them a heart of flesh, 20 that they may walk in my statutes and keep my rules and obey them. And they shall be my people, and I will be their God.
(Eze 36:26-27 ESV) 26 And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. 27 And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules.
Ezekiel repeatedly speaks of the end of the Exile as a time when God replace Israel’s heart of stone with one of flesh and God’s Spirit. The parallels with Deu 30:6 should be obvious. Thus, Ezekiel identifies the Spirit as God’s means of circumcising hearts.
(Eze 37:11-14 ESV) Then he said to me, “Son of man, these bones are the whole house of Israel. Behold, they say, ‘Our bones are dried up, and our hope is lost; we are indeed cut off.’ 12 Therefore prophesy, and say to them, Thus says the Lord GOD: Behold, I will open your graves and raise you from your graves, O my people. And I will bring you into the land of Israel. 13 And you shall know that I am the LORD, when I open your graves, and raise you from your graves, O my people. 14 And I will put my Spirit within you, and you shall live, and I will place you in your own land. Then you shall know that I am the LORD; I have spoken, and I will do it, declares the LORD.”
Ezekiel’s powerful vision of the valley of dry bones finishes with a promise to give life. “I will put my Spirit within you, and you will live.” Again, the “you will live” is an allusion back to Deu 30:6 (“that you may live”) — and this is why, in Rom 8, Paul refers to the Christian’s receipt of the Spirit as the “Spirit of the law of life.” It’s by the Spirit that our hearts are circumcised (made soft, made obedient), that we are caused to obey, and we live.
There are many more such verses, and so we see that Paul did not invent the idea of the Spirit being the means of circumcising hearts. That comes primarily from Ezekiel, but other prophets as well. Indeed, according to Jeremiah, it’s the foundation of the new covenant —
(Jer 31:31-34 ESV) “Behold, the days are coming, declares the LORD, when I will make a new covenant with the house of Israel and the house of Judah, 32 not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the LORD. 33 For this is the covenant that I will make with the house of Israel after those days, declares the LORD: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people. 34 And no longer shall each one teach his neighbor and each his brother, saying, ‘Know the LORD,’ for they shall all know me, from the least of them to the greatest, declares the LORD. For I will forgive their iniquity, and I will remember their sin no more.”
Just as Moses said in Deu 30:6, the change in covenant will be based on God himself changing the hearts of this people. Obedience will come because of the direct operation of God on the hearts and minds of his people.
V. 34 promises forgiveness, but this is because God’s people will know God and God will have changed their hearts, through the Spirit (according to Ezekiel).
An important, uninspired book written between the testaments is the Book of Jubilees —
1:22 And the Lord said unto Moses: ‘I know their contrariness and their thoughts and their stiffneckedness, and they will not be obedient till they confess their own sin and the sin of their fathers. 23 And after this they will turn to Me in all uprightness and with all (their) heart and with all (their) soul, and I will circumcise the foreskin of their heart and the foreskin of the heart of their seed, and I will create in them a holy spirit, and I will cleanse them so that they shall not turn away from Me from that day unto eternity. 24 And their souls will cleave to Me and to all My commandments, and they will fulfil My commandments, and I will be their Father and they shall be My children. 25 And they all shall be called children of the living God, and every angel and every spirit shall know, yea, they shall know that these are My children, and that I am their Father in uprightness and righteousness, and that I love them.
This book is generally considered to have been written in the Second Century BC, that is, at least 100 years before Jesus, and yet the author had already combined Deu, Jer, and Eze to combine the circumcision of hearts and the coming of the Spirit and forgiveness of sin.
So forgiveness of sin was not seen as a requirement for entry into the Kingdom by itself. It was necessary, but it would come from receipt of the Spirit, which would circumcise/soften hearts to cause the people to be obedient.