Since righteousness comes by faith alone, it leads us to ask the question, why circumcision? Paul did not keep us guessing at the answer. Circumcision was the sign of the what had taken place within the heart, therefore the physical evidence of being circumcised was a token of that change. It was a seal signifying the covenant.
Today we have a perfect example of this. During a marriage ceremony the couple exchanges wedding rings as a token of their love and devotion to each other. The rings signify that both parties are entering into a lasting covenant before God. Does wearing a wedding ring mean you are married? No, the commitment to each other and vows you took, means you are married. The ring however, is a physical identification that the person has entered into the marriage covenant. It is a symbol both, of what has taken place and it serves as a reminder of the promise made.
Circumcision is also a symbol of what takes place within the heart. Abraham was justified by faith because he believed God. Because of it, God credited Abraham with righteousness. Circumcision was the sign of the covenant between Abraham and God. It was a physical token of what took place privately.
The good news for us today is, that because Abraham had been credited with righteousness before he was circumcised, righteousness is also credited to all who believe. The scriptures say that "Abraham believed God, and it was credited to him as righteousness." ( Romans 4:3 ). So if we have the same faith that our father Abraham had, we too are justified by it. Today, I am so glad that God remembers the details. I rejoice, that by Abrahams example, we can know for sure that we are righteous before God by faith in the blood of Jesus.