OLD TESTAMENT LAW!
God made a covenant with Abraham and all his descendants to be circumcised (cutting off the foreskin) as a sign to the rest of the world.
GENESIS 17:10-11 "This [is] My covenant which you shall keep, between Me and you and your descendants after you: Every male child among you shall be circumcised; 11 "and you shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between Me and you. 12 "He who is eight days old among you shall be circumcised, every male child in your generation."
JESUS WAS CIRCUMCISED ACCORDING TO THE LAW.
LUKE 2:21 And when eight days were completed for the circumcision of the Child, His name was called Jesus, the name given by the angel before He was conceived in the womb.
NEW TESTAMENT GRACE!