Actually, the foundation of Christian ethics is the reciprocal law-
The sort of reciprocal altruism encouraged by Jesus stands in stark contrast to the morale absolutism predominant in the Old Testament.
Actually the greatest moral ethics for a Christian is stated thus.
Matthew 22:35-40 Then one of them, which was a lawyer, asked him a question, tempting him, and saying,
36 Master, which is the great commandment in the law?
37 Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.
38 This is the first and great commandment.
39 And the second is like unto it, Thou shalt love thy neighbour as thyself.
40 On these two commandments hang all the law and the prophets.
These two great commandments are also found in the Old Testament. The first great commandment is found in Deuteronomy 6:4-9. This is known as the Mishna. This commandment was something that the Israelis were meant to recite everyday so that they would remind them that their first duty was to love God, no matter what they did.
The second commandment is found originally in Leviticus 19:18. Basically these two commandments are what the law is about about. You can condense that whole 613 different laws in the Torah into these two commandments. ALso the Ten commandments can be broken down into these two great commandments, with the first four Commandments about our duty to God (Having no other gods in place of Jehovah, no graven images, no bowing down to Idol and keeping the Sabbath and keeping it Holy.) The last six refer to our duty to our fellow human (Honour our parents, do not murder do not commit adultery, not stealing, do not bear false witness against others, do not covet things that are not yours.)
And every single one of these laws were repeated in the New Testament except the keeping of the Sabbath, but that does not mean that in the New Testament that there was not a day where people would specifically rest and worship God, that Being the first day of the week, Sunday. This was the only change in the Ten Commandments since now the holiest day for a Christian is the day that Jesus rose from the dead and each Sunday we are remembering that occasion.
What people are missing about the Old Testament is the fact that is shows us that we are totally incapable of keeping God's Commands. We see throughout the Old Testament people forsaking God's commands, even though they should be keeping it. That is the story of History. The law shows us that we are wicked and that it all that it can do.
Galatians 3:22-26 But the scripture hath concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.
23 But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed.
24 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith.
25 But after that faith is come, we are no longer under a schoolmaster.
26 For ye are all the children of God by faith in Christ Jesus.
John 1:17 For the law was given by Moses, but grace and truth came by Jesus Christ.