"Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfill." Matthew 5:17
Christ takes us through and behind the Law of the Old Testament and, if we are to make a sincere study of His entire sermon (Matthew 5, 6, and 7) we will realize that obedience to Christ's teaching is far more difficult than adhering to the Commandments.
You may wonder why, especially if you have been taught that it is "by grace that we are saved". Eph 2:8 Indeed, but are we to disregard this convicting Sermon?
There is a vast difference between the kind of obedience to the Old Law, and what Christ requires of us. The Old Law is a litany of "shall not" sins.
Christ requires that we possess a condition of our hearts such that we not even think of sins. Our thoughts and imaginations of heart and mind must be blameless.
Now we are bewildered, maybe even disheartened, because, in reality, who is able to not sin in heart and mind? Not one of us. We are not pure and holy. What is the use then? Where are we to begin? Is Christ saying we must never lust, never hate, always have a forgiving disposition, never lie? Well, then, why even try?
We are thinking much like the disciples did when they witnessed Christ's encounter with the rich young man. They said to one another, "Who then can be saved?"
"With men it is impossible, but not with God: for with God all things are possible."
It is because Christ died for His own, and resurrected, that we are forgiven of our sins, and that the elect will have everlasting life and live eternally with Him.