This is vital to understand when scripture says believers are unable to sin, but still do sin.
We are comprised of 3 parts: we are a soul that has a spirit and lives in our fleshly bodies.
There is also the concept that we have a heart and a mind. However, we are not comprised of 5 parts: a soul, spirit, body, heart and mind. The heart and the mind are synonymous — the same thing. When you, “believe with the heart”, it’s the same thing as, “believing with the mind.”
So if our makeup is only a soul, spirit and body, where does the heart/mind fit in? Let’s break this down: we know that in order to be saved, we must believe with our heart/mind the gospel found in 1 Corinthians 15:1-4 (that Christ died for our sins, was buried and rose again on the third day according to the scriptures).
We also know that we are born spiritually dead. So if the heart/mind were part of the spirit, it wouldn’t make sense because how does a dead spirit believe in the gospel? This can be backed up in 1 Corinthians 2:11 — “For who among men knows the thoughts of a man [his soul] except the spirit of the man which is in him? Even so the thoughts of God no one knows except the Spirit of God.” So you can see that the thoughts of a man are related to the man himself – his soul – and that his spirit knows his soul’s thoughts.
Let’s talk about the heart/mind possibly belonging to the fleshy body. We know that our fleshy bodies contain a fleshy brain, which in itself has thoughts, desires and emotions for the body – which can compete with our soul (and oftentimes overwhelms us). We also know that upon believing, our fleshy bodies are not renewed, but stay fleshy and sinful, and only at the time of the rapture/resurrection will our bodies be transformed into our heavenly, spiritual, glorified, sinless bodies (1 Thessalonians 4:16-18). So does it make sense that you, as a soul, would have to have your corrupted, sinful fleshy body that will be done away with, believe the gospel with its fleshy brain in order for the soul to be saved and be spiritually reborn? No.
So the only part of our makeup that makes sense that the heart/mind would be a part of is the man himself – the soul. I’m sure you’ve heard the saying: “I believe with all my heart, mind and soul.” When a man – as a soul – hears the gospel and believes it, he is spiritually born again and he – as a soul – becomes a new creature in Christ. A man must be spiritually reborn: John 3:5 — “Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.” A man who believes in Christ is a new creature: 2 Corinthians 5:17 — “Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.” Our souls have been purified upon believing the gospel and we have been born again of incorruptible seed: 1 Peter 1:22-23 — Seeing ye have purified your souls in obeying the truth through the Spirit unto unfeigned love of the brethren, see that ye love one another with a pure heart fervently: Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever. If our soul is made anew, then so our heart/mind – being a part of the soul – is made anew also: Hebrews 8:10 — “For this is the covenant that I will make with the house of Israel after those days, says the Lord: I will put My laws in their mind and write them on their hearts; and I will be their God, and they shall be My people.”
Thus, our “inner man” – us a soul (containing our heart/mind) and our spirit – are renewed when we believe, and cannot sin as it is kept by Christ from stumbling. However, our body as sinful flesh is not renewed upon believing, and still desires to sin. When we get our new, glorified, uncorrupted spiritual bodies at the rapture/resurrection, then we will be completely sinless.
Paul’s struggle with sin
In Romans 7:14-25, Paul describes his struggle with sin. However, he makes it clear that his inner man/soul/mind delights in the law (Hebrews 8:10), but his flesh still desires to sin. Therefore, it is not Paul (his “inner man”, his soul) that sins, it is his fleshy, unregenerate body that sins:
Romans 7:14-25 — For we know that the law is spiritual, but I am carnal, sold under sin. For what I am doing, I do not understand. For what I will to do, that I do not practice; but what I hate, that I do. If, then, I do what I will not to do, I agree with the law that it is good. But now, it is no longer I who do it, but sin that dwells in me. For I know that in me (that is, in my flesh) nothing good dwells; for to will is present with me, but how to perform what is good I do not find. For the good that I will to do, I do not do; but the evil I will not to do, that I practice. Now if I do what I will not to do, it is no longer I who do it, but sin that dwells in me [the flesh]. I find then a law, that evil is present with me [his soul], the one who wills to do good. For I delight in the law of God according to the inward man. But I see another law in my members, warring against the law of my mind, and bringing me into captivity to the law of sin which is in my members [flesh]. O wretched man that I am! Who will deliver me from this body of death? I thank God—through Jesus Christ our Lord! So then, with the mind I myself serve the law of God, but with the flesh the law of sin.
Our inner man is kept from stumbling
Jude 1:24 — Now to Him [Jesus] who is able to keep you from stumbling, And to present you faultless Before the presence of His glory with exceeding joy,
A believer cannot sin as the inner man
1 John 3:9 — Whoever has been born of God [the inner man] does not sin, for His seed remains in him; and he cannot sin, because he has been born of God.
Our flesh still sins
1 John 1:10 — If we say that we have not sinned, we make Him a liar, and His word is not in us.
Our new bodies at the rapture/resurrection
1 Corinthians 15:50-53 — Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; nor does corruption inherit incorruption (if our soul was still corrupted upon believing the gospel, it could not enter heaven). Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed— in a moment, in the twinkling of an eye, at the last trumpet [the rapture/resurrection]. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed. For this corruptible [body] must put on incorruption, and this mortal [body] must put on immortality.
Who knows the mind of a man but his spirit
1 Corinthians 2:11 — For what man knoweth the things [thoughts] of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.
Born of incorruptible seed and soul purified by faith
1 Peter 1:22-23 — Seeing ye have purified your souls in obeying the truth through the Spirit unto unfeigned love of the brethren, see that ye love one another with a pure heart fervently: Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever.