The law, specifically containing regulations about circumcision, festival days and dietary laws, was given at a time when the people of God were young. There needed to be some way to identify the people of God. It certainly would not be by their ethical behavior because it was abysmal.
In the family, a child can be hardly distinguished from a slave. He is subject to others, older. But there comes a time when the child is no longer a child and must act with the responsibility of an adult – more importantly, an heir.
From the end of chapter three of Galatians through chapter four, we find some difficult comparisons, but the heart of the matter is this: If being a child of God is just a matter of keeping the rules, then the child is simply and solely a child. The adult keeps the rules not because they are rules, but because it is his way of life. A child is always breaking the rules, and as such, demonstrates a slavery to influences outside his heritage. But the adult is true to his heritage and behaves as he should simply because it is his nature.
If we seek to reduce Christianity to a few litmus tests of righteousness, we will still be acting like slaves – children. We will always be controlled by something other than the Spirit of God – our real DNA. But when we give up litmus tests and simply seek to follow Jesus, we will be adults and our lives will be noticeably different.