The primary purpose of the new covenant was not for God to come and change our external behaviour. Rather it was God changing the way He relates to us.
Under the Old Covenant, God dealt with the external problem, namely people’s sins. They were punished and cursed when they sinned and broke the laws. But under the New Covenant, God dealt with the internal problem, namely our fallen sinful nature. He changed us from the inside, giving us the very nature of his Son, Jesus Christ, and credited the perfect obedience of Jesus to our account.
Our old sinful nature has been crucified with Christ, and our spirits made 100% righteous:
“For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him”. (2 Cor 5:21, NKJV)
And this is the way He now sees us: blameless and fully righteous in His sight:
21 And you, who once were alienated and enemies in your mind by wicked works, yet now He has reconciled 22 in the body of His flesh through death, to present you holy, and blameless, and above reproach in His sight. (Col 1:21-22, NKJV)
6 knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. 7 For he who has died has been freed from sin. (Rom 6:6-7, NKJV)
Before we came into Christ, we were enemies of God, and we were alienated from Him in 2 ways:
1) We had a sinful nature inside us and we were slaves to this sinful nature, running after every evil thing to gratify the desires of our flesh:
“among whom also we all once conducted ourselves in the lusts of our flesh, fulfilling the desires of the flesh and of the mind, and were by nature children of wrath, just as the others”. (Eph 2:3, NKJV)
We were alive to sin (because of our sinful nature) and dead towards God.
2) We were also under the law, which utterly condemned us and showed us how sinful we were.
19 Now we know that whatever the law says, it says to those who are under the law, that every mouth may be stopped, and all the world may become guilty before God. 20 Therefore by the deeds of the law no flesh will be justified in His sight, for by the law is the knowledge of sin. (Rom 3:19-20, NKJV)
But Rom 6:6-7 says that we are now dead to sin. We are now friends of God, and we were united with Him in 2 ways:
1) Our sinful nature was crucified with Christ, and our spirits have now been made alive towards God. The perfect righteousness of Jesus Christ has been imputed to us, and all the benefits and blessings of His obedience has now been credited to us because of our faith in Him.
2) The law has been nullified, nailed to the cross with Jesus:
“having wiped out the handwriting of requirements that was against us, which was contrary to us. And He has taken it out of the way, having nailed it to the cross”. (Col 2:14, NKJV)
God now relates to people again like He did in the days of the Bible before the law was given, not imputing their sins against them, but instead delighting in us with the love and passion of a proud Father. God doesn’t just see us as if we’ve never sinned – He sees us now as if we have perfectly obeyed all the laws our entire life!
4 But God, who is rich in mercy, because of His great love with which He loved us, 5 even when we were dead in trespasses, made us alive together with Christ (by grace you have been saved), 6 and raised us up together, and made us sit together in the heavenly places in Christ Jesus. (Eph 2:4-6, NKJV)