Why Do Christians Adhere to Some Old Testament Commands and Not Others?
This phrase, or something like it, is common to hear. Many Christians are thrown off by it. A common (but wrong) Christian response is to say that because that is the Old Testament, we don't follow it anymore (just the New Testament). But this brings up the obvious question as to why we still reference the 10 Commandments (part of the Old Testament).
The true answer is that Christians neither reject nor follow all the laws in the Old Testament.
The Old Testament law is all good. Jesus said, in Matthew 5:18, "For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished." So, the law continues, but the law also has various purposes to it.
Broadly, there are three general categories of laws. The Moral law (found most clearly in the 10 commandments but in many other places of the OT), the Civil Law (laws relating to how the government should punish sins/crimes), and the Ceremonial law (laws relating to the temple and how to be ceremonially clean so that you should open it).
In Christ, two things happened. First, the nation of Israel (with swords, courts, and soldiers) ceased to be the ruling power over the people of God. Christians were sent not as a nation but as colonies of "citizens of heaven" to live in the unbelieving nations. Therefore civil laws (such as how to punish crimes and sins) are not to be exercised by us as we live among the unbelieving nations. Second, Jesus replaced the temple as the perfect sacrifice. Laws relating to how to enter the temple (washing, kosher foods, and etc) no longer make sense given that we are now always "in Christ" (the true temple).
This means leaves us with the Moral Law. How to live in a way that pleases God. How we worship. How we love our neighbors. How we behave ethically in regards to business, sex, and family.
And this part of the law remains central to how Christians are called to live. The moral law is summarized well in the Ten Commandments. This is why Christians continue to teach our kids these commandments. Reflect on these commandments. Post these commandments. And this is why Christians might point to an Old Testament passage to explain why homosexual behavior is not according to Christian ethics but we do not point to ceremonial laws to learn how to eat. And we do this without inconsistency or hypocrisy.