"Cross of Jesus, Cross of Sorrow"

The last lines of "Cross of Jesus, Cross of Sorrow" are "Perfect man on thee did suffer, / Perfect God on thee has bled!"  These lines have a parallel structure, and this is used to indicate that Jesus is both fully man and fully God.  As Luther puts it in the explanation to the second article of the Apostles' Creed, Jesus is "true God, begotten of the Father from eternity, and also true man, born of the Virgin Mary."