Question Number 994:
I am confused about how to understand the Old Testament Mosaic law. What was the real purpose of it? Why do we keep some of it, for example the laws concerning sexuality, but not the others such as the laws of uncleanliness, mixing fibers etc.How much are we expected to keep as Orthodox Christians?
The purpose of the Law was to reveal sin and to be, for the Jews, a tutor leading to Christ. Its purpose has been fulfilled.
Christians are not under the Law of Sinai, in the same way that US citizens are not under British law. The two might often be similar, but there are simply not the same. There are commandments that are binding upon Christians - it is those that are repeated in the New Testament. This includes sexual commandments as well as ethical commandments. But it would be dangerous and wrong to see Christianity as an adjustment to the Old law. Christianity is new way to exist - a renewed heart and spirit (the gift of God) which leads someone to be like God and to perform every act with love, not because of a law or set of commandment, but because this is the way the person is.
In summary, Christians are not expected to keep anything found in the Old Testament Mosaic Law. None; because we are not bound by it. We are expected as Christians to obey the Lord's commandments and all the commandments set forth in the New Testament, but it is an obedience that flows from a renewed heart and spirit. Of course, not being bound by the Mosaic Law does not mean that we are not informed by it.
Let's take a concrete example. A woman has a child and the Orthodox custom is to stay at home for 40 days, a custom that is informed by the Old Testament. Let's say that this Orthodox woman is feeling great and wants to return to Church after 20 days. Should she do that or is she bound that the Mosaic model? The answer suggested here is that she is not bound that the Mosaic model but that she is bound by obedience to the leadership of the Church (Hebrews 13; Matthew 18; Acts 15). If the bishop says, "sure, you are blessed to return" then she may; but if he says "I want you to stay home according to the wisdom of this tradition," she should stay home on account of obedience and humility.