It’s really quite simple. The Old Covenant was intended to make a person right in the eyes of God by his adherence to the Law. But this is not the same as relationship with and adherence to God, Himself. With the Old Covenant, we must demonstrate our righteousness, by our own, external, efforts, as if we possess and control that righteousness on our own. The Jews in the Old Testament failed at this endeavor over and over, as we all inevitably will because we possess no real righteousness on our own. The real purpose of the Old Covenant, ultimately, was actually to
prove that we cannot do it on our own, that we need God, a
communion with Him that we were
made for, in order to accomplish this, in order to become righteous
interiorly, first of all; in order to be fully and truly who we were created to be, to put it another way. Adam thought otherwise and effectively broke communion with God, as he became his own “god” for all practical purposes. And this is the state of being, spiritual separation from God (aka the “death of the soul”), that all men are born into now. So both covenants demand righteousness from man but the old could not accomplish what the new can. The New Covenant mantra
“Apart from Me you can do nothing”, Jesus speaking in John 15:5.
So the first marching order with the New Covenant isn’t to obey the commandments, but rather to
enter communion with God, via faith professed and confirmed by Baptism, Who, alone, can give new life and cause authentic obedience in us, Who can
justify us IOW. Thus, the most important New Covenant Prophecy, from Jer 31:
“I will put my law in their minds
and write it on their hearts.
I will be their God,
and they will be my people." Jer 31:33
and it continues:
No longer will they teach their neighbor,
or say to one another, ‘Know the Lord,’
because they will all know me,
from the least of them to the greatest,”
declares the Lord.
“For I will forgive their wickedness
and will remember their sins no more.” Jer 31:34
This knowledge is the object of faith, the direct “knowledge of God” that Jesus came to reveal. This speaks of something different now, something changed; it speaks of communion or relationship, even if that knowledge is only partial here and not fully consummated until the next life (1 Cor 13).
"Now this is eternal life: that they know you, the only true God, and Jesus Christ, whom you have sent" (John 17:3). Knowledge of the true God produces love of God.
The Church teaches that the New Covenant doesn’t
revoke the Old; it simply replaces it with a new and better one, when the time was ripe in human history, when man was finally becoming able to receive that light. Simply put: man needs grace; man needs
God.