21 “My hand shall hold Him fast, and My arm shall strengthen Him”: because there was a taking up of man; because flesh was assumed in the Virgin's womb, because by Him who in the form of God is coequal with the Father, the form of a servant was taken, and He became obedient unto death, even the death of the Cross.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)