I am inclined to agree with @Sonny Childs when he says, “there should be little debate that ‘spirit’ refers to God’s nature,” The Bible says, “…be transformed by the renewing of your mind” (Romans 12:2). How is the mind renewed? By studying God’s Word (the Bible) or by “another entity” dwelling in our bodies, taking control of our minds and renewing it? Wouldn’t the second premise be similar to a maker reprogramming a robot?
We struggle with the Spirit often because we ignore the OT background. Paul in Rom 8 makes far better sense if you’ve read the Law and Prophets. Let me offer a very abbreviated introduction. You can easily dig out the rest with a concordance and cross-referencing Bible.
We start with —
(Deut. 10:16-20 ESV) 16 Circumcise therefore the foreskin of your heart, and be no longer stubborn. 17 For the LORD your God is God of gods and Lord of lords, the great, the mighty, and the awesome God, who is not partial and takes no bribe. 18 He executes justice for the fatherless and the widow, and loves the sojourner, giving him food and clothing. 19 Love the sojourner, therefore, for you were sojourners in the land of Egypt. 20 You shall fear the LORD your God. You shall serve him and hold fast to him, and by his name you shall swear.
which takes us to
(Deut. 30:6 ESV) 6 And the LORD your God will circumcise your heart and the heart of your offspring, so that you will love the LORD your God with all your heart and with all your soul, that you may live.
God commands Israel to change their hearts from stubborn to loving God with all their heart and soul. Later, he prophesies that there will come a time when God himself changes (circumcises) their hearts. This is not a prophecy of God’s word coming to them, as the Israelites had the Law of Moses and were told to get their hearts right (circumcised). The difference is that GOD HIMSELF would change their hearts.
(Jer. 31:31-34 ESV) 31 “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.”
This passage is where we get the term “new testament.” “Testament” is an archaic term for covenant. God says under the new covenant, God himself will write his laws on the hearts and minds of his people. The change will be God doing the writing — to change hearts.
This passage is quoted in full in Heb 8 and is applied to the new covenant instituted by Jesus.
To similar effect is —
(Jer. 24:7 ESV) 7 I will give them a heart to know that I am the LORD, and they shall be my people and I will be their God, for they shall return to me with their whole heart.
Again, the emphasis is on the heart. God himself will change hearts.
(Ezek. 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 then promises “a new heart” and God’s “Spirit within you,” the result being obedience.
Now, to imagine that Moses, Jeremiah, and Ezekiel are speaking of the Bible alone is absurd. The point is that God’s written word is not enough – not until God himself enters our hearts and changes us. This is NOT Calvinism. It’s Moses and the Prophets, and the passages don’t say that God will overcome our free will. Rather, they say our hearts will be softened so we can love God as we should and so “live” (Deu 30:6 – echoed throughout these passages).
This leads to —
(Rom. 2:25-29 ESV) 25 For circumcision indeed is of value if you obey the law, but if you break the law, your circumcision becomes uncircumcision. 26 So, if a man who is uncircumcised keeps the precepts of the law, will not his uncircumcision be regarded as circumcision? 27 Then he who is physically uncircumcised but keeps the law will condemn you who have the written code and circumcision but break the law. 28 For no one is a Jew who is merely one outwardly, nor is circumcision outward and physical. 29 But a Jew is one inwardly, and circumcision is a matter of the heart, by the Spirit, not by the letter. His praise is not from man but from God.
Paul is echoing the prophets — and declaring that the Gentiles who love God and obey his will obviously have had their hearts circumcised by the Spirit, as Moses had prophesied for the true Israel. And if you, even if a Jew, don’t obey God, you don’t have circumcision of the Spirit — and so your physical circumcision is of no value.
Which brings us to —
(2 Cor. 3:5-8 ESV) 5 Not that we are sufficient in ourselves to claim anything as coming from us, but our sufficiency is from God, 6 who has made us sufficient to be ministers of a new covenant, not of the letter but of the Spirit. For the letter kills, but the Spirit gives life. 7 Now if the ministry of death, carved in letters on stone, came with such glory that the Israelites could not gaze at Moses’ face because of its glory, which was being brought to an end, 8 will not the ministry of the Spirit have even more glory?
Paul doubtlessly had Deu 10 and 30 in mind. The “ministry of death, carved in letter on stone” is the Law of Moses — which commanded the Jews to circumcise their own hearts. It failed.
But the “ministry of the Spirit” in which God softens our hearts and writes his laws on our hearts so that we obey is of far greater glory.
“The Spirit gives life” refers to the promise in Deu 30:6 that if God circumcises our hearts for us, we will live