To elaborate on what has been discussed in previous posts, as Christians (or, at least, as Catholics, unfortunately it is hard to make any statements beyond very general ones that represent the beliefs of all Christians) we believe that when God revealed Himself to man within the history of time, He did so gradually, only revealing as much of Himself and His nature as man was ready to accept at that particular time. So in the Old Testament, we see a gradual revalation from Adam to Noah to Abraham to Moses and to David, with each subsequent revalation providing a bit more information to add to the puzzle than the last. This gradual revelation culminated when God Himself (that is, God the Son, the Second Person of the Blessed Trinity) became man in the person of Jesus Christ (whom we believe is both fully God and fully man) and delivered to humanity the fullness of His revelation.
To illustrate the above, consider the following example. In the time of Moses, it was not uncommon for one who was injured by another to retaliate in a manner that exceeded the initial injury (i.e. a man who had his eye gouged out by another may have retaliated by killing the other man, a retaliation which clearly exceeds the initial injury). Therefore, to limit such excessive retaliation God commanded through Moses “Anyone who inflicts a permanent injury on his or her neighbor shall receive the same in return: fracture for fracture, eye for eye, tooth for tooth. The same injury that one gives another shall be inflicted in return.” (Leviticus 24:19-20) In other words, the retaliation or punishment was to correspond in kind and degree to the initial injury and no more. At the time of Moses, man was not yet ready to accept the fullness of the revelation that would come through Jesus. However, when Jesus came along, He showed us a still more excellent way: “You have heard that it was said, ‘An eye for an eye and a tooth for a tooth.’ But I say to you, offer no resistance to one who is evil. When someone strikes you on (your) right cheek, turn the other one to him as well. If anyone wants to go to law with you over your tunic, hand him your cloak as well. Should anyone press you into service for one mile, go with him for two miles. Give to the one who asks of you, and do not turn your back on one who wants to borrow.” (Matthew 5:38-42) Here we see that the fullness of God’s revelation is not only to not retaliate in an excessive manner, but to not retaliate at all. Therefore, what had been the practice under the Mosaic law no longer applies because what was only partially revealed at the time of Moses is now fully revealed in Jesus.
I hope that explantation and example helps somewhat.