Jesus explains in the NT and revealed several times in the OT that we cannot change the sinfulness of our hearts, "Not that which goeth into the mouth defileth a man; but that which cometh out of the mouth, this defileth a man." The motivation for sinful behavior is found in the heart, "That whosoever looks on a woman to lust after her has committed adultery with her already in his heart."
Some Christians are pietists (Amish) believing that their actions will make them holier than other Christians. Some are legalists (could be Amish) and seek to add to the finished work of Christ for their salvation with their actions. What we find consistently revealed in scripture is the inability of man to effect any change in himself and the need for the redeeming blood of Christ.
The former covenants, in many ways, teach us that we cannot fulfill the obligations contained within them. It is only by Jesus Christ and Him alone that anyone is ever saved. Why is that? He fulfilled our covenant obligations for us and we receive His active obedience through imputation. Our sin was imputed to Him and He died in our place to fulfill the debit we could not pay.
The new covenant is described in Jer. 31.31, 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. That is the only thing that matters...a new heart.
This new covenant of grace is marked by the Law being written on the heart and heart circumcision. This heart circumcision was typified in Mosaic covenant with physical male circumcision and has been replace in total with a better covenant. (Heb. 8) The very heart of the Gospel is salvation by Christ alone...through faith. That includes sanctification as well as justification. I always frame the Gospel in this manner to place emphasis on the efficacy of Christ's covenantal work and His surety-ship on our behalf. This is the ontological order by which a sinner is saved even though modern evangelicalism often places the emphasis on faith first and the work of Christ second.
I'm rambling now...
It doesn't matter if you are Jewish or Amish, whatever...it's the new birth that matters.
Yours in the Lord,
jm