Answer: Circumcision was the sign of a covenant God made with Abraham and his descendants (Genesis 17:9–14; Acts 7:. The Mosaic Law repeated the requirement that all males be circumcised (Leviticus 12:2–3). So every Israelite boy, as well as any man desiring to become part of the Hebrew people, was circumcised (Exodus 12:48). Since Jesus was a Jew living under the law, He would have been circumcised on the eighth day as were all male Hebrew babies.

Luke 2:21 records the fact that Jesus was circumcised: “On the eighth day, when it was time to circumcise the child, he was named Jesus, the name the angel had given him before he was conceived.” Later in the same chapter, Luke emphasizes that Joseph and Mary followed all the Jewish requirements, doing “everything required by the Law of the Lord” concerning their newborn son (verse 39). In following the law, Joseph and Mary would undoubtedly have circumcised Jesus. Failure to do so would have been a clear violation of the law.

Jesus spoke in the synagogues and taught in the temple courts in Jerusalem (Luke 4:16; 19:47). If He had been uncircumcised, Jesus would have been excluded from those activities. He would not have been allowed inside those areas.

Later in His ministry, Jesus said, “I always do what pleases [the Father]” (John 8:29). Jesus could not have been fully pleasing to God if He had not been circumcised, because disobedience to the law cannot please the Lawgiver. One purpose of Jesus coming to earth was to fulfill the law (Matthew 5:17–18). He, as a Man, lived in perfect obedience to all God had decreed for humanity. In doing so, His life was without spot or blemish and completely acceptable to God. Only as the perfect sacrifice could Jesus provide atonement for sin (Leviticus 4:32; Hebrews 9:14; 1 Peter 1:19).

We know that Jesus was circumcised because, in that day, God required it. The law has been fulfilled. Circumcision is not a requirement for the church and has nothing to do with one’s standing before God (Galatians 2:16).

Why not? Wasn't He born a Jew? See question

Galatians

5:6
For in Christ Jesus neither circumcision nor uncircumcision has any value


6:14-15
May I never boast except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world. Neither circumcision nor uncircumcision means anything; what counts is the new creation