Question: Why don't Christians today obey all the laws and instructions of the Old Testament?
The short answer is, we did. That needs to be explained, doesn't it?
Leviticus 19. Israel is not "one nation under God," it is The Nation Ruled by God, and here is what that looks like: "You shall be holy, for I the LORD your God am holy." You shall be different than those other nations around you that are not The Nation Ruled by God. You shall be a nation set apart to me. Count how many times chapter 19 reminds them, "I am the LORD your God." This long list of instructions tell the Israelites how to do "I am the LORD your God." The chapter closes simply and straightforward: "Do them!"
Jesus did them. "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them" (Matthew 5:17). He fulfilled all righteousness, then offered the most incredibly, ridiculously wonderful trade of all time: his righteousness for our sin. "For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God (2Corinthians 5:21)."
This doesn't mean the Law goes away. Jesus repeats, "Be holy, for I the LORD your God am holy" (Matthew 5:48, he says 'perfect,' but means the same thing), and so do his disciples (1Peter 1:16). The key is a verse right in the middle of Leviticus 19, "You shall love your neighbor as yourself." Jesus, who did it all, says that verse sums it up: all the Law and the Prophets hang on two commandments: Love God, love your neighbor (Matthew 22:40).
We obeyed the Law when Jesus obeyed the Law, and when his righteousness became ours. Now we live it. Jesus said, "You shall love the Lord your God with all your heart and with all your soul and with all your mind. This is the great and first commandment. And a second is like it: You shall love your neighbor as yourself. On these two commandments depend all the Law and the Prophets." ~Matthew 22:37-39
God, You so loved the world that You sent Your one and only Son. Anyone believing in Him will receive eternal life. In Your grace, You did not send Your Son to condemn me. In Your holiness, You would have been justified to do so, but Your Son became condemnation so I might receive justification in His Name.
The Name of Jesus is the Name above all names. May I never mix it with any others. May I always hold it high. May I walk in Your ways, increasing Your reputation in this world. May I humble myself and direct all glory to You.