There have been a lot of people who have a complaint between the apparent contradiction between Law and Grace. How can it be the same God that says "Repay blood for blood (Genesis 9:6)" and then "Love your enemy (Matthew 5:44)" but truly, I say to you. They are one and the same.
Both Law and Grace are covenants. Law is the covenant of the physical realm, the covenant with the world that was made with Abraham (who was brought out of the world [Ur] into God's Kingdom [Israel]). This was the covenant detailing two different aspects.
A) What the new nation, Israel, would do to separate itself as a nation.
B) What the people of God were to act like.
Now, within the covenant of Law, you have three aspects.
A) The Jacobic or National aspect, laws such as "Don't cut your beard (Leviticus 19:27)" to "Build roadways to the Cities of Refuge (Deuteronomy 19:3)".
B) The Ritual aspect, laws such as the sacrificial system (Leviticus 1-17) and priesthood (Exodus).
C) The righteousness aspect, laws such as the 10 commandments (Exodus 20).
However, when the covenant of Grace was signed with the Blood of Christ with all those who would have faith in the ressurected Christ. This covenant was signed by God with the death of his son, and this covenant is singed by us with the death of ourselves (full-immersion water baptism).
Christ was the last sacrifice (Hebrews 10:10), thus nullifying the sacrificial system, and he became our High Priest (Hebrews 4:14), thus nullifying the priesthood.
In addition, through Christ's death the Nation of Israel became a heavenly nation, not one of this world (John 18:36). With Christ being it's King. And we, as gentiles, are grafted onto the tree of "Israel" (Romans 11). And therefore, the Jacobic laws have been abolished because of a changing of God's Chosen Nation.
But then the question arises, why did God put the Law in place anyways? Well, the law was a foretaste of what was going to happen. (Hebrews 10:1) And Jesus was the final culmination of the Law. The Law became complete in Christ, it became a relationship as opposed to a set of rules. All Christians should follow The Law, but because Jesus is now The Law.
If there's anything I haven't explained well, comment and I will add to this chapter.
“For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him.”