We have said that the New Testament is a fulfillment of the Old. This is true both of Christ's teaching of what we are to believe and of Christ's telling us how we are to behave. Our emphasis, of course, is on Christian behavior which among the followers of Christ is to be more perfect than was ever the conduct even of the Chosen People under the law of Moses. The inwardness or interiority of Christian behavior is spelled out by the Savior through most of His Sermon on the Mount. He brings out the stark contrast between the laws that were given to Moses as compared with the conduct of His own followers.
True morality is never merely external.
It either has interior motivation or it is not worthy of the name. Yet, as the Master points out, the children of Abraham were to practice numerous external prescriptions as a witness of their fidelity to Yahweh. The hundreds of precise prescriptions in the book of Deuteronomy alone are evidence of how exacting the Mosaic law could be. Days and seasons, amount and kind, gender and age, length and quantity, were specified in the extreme. All of this was part of God's will for His people to keep them united to Himself and united among themselves as the Chosen race.