Assuming the whole nature of humanity

He who reveals God to man, and reconciles man to God, must be both God and man, truly and completely God, and truly and completely man. If the Son was to redeem the whole nature of man, he had to assume the whole nature of man; if in the Son man is to be gathered into the fellowship and life of God, it must be by one who is truly and completely God. Only he can be mediator who is himself the union of God and man, only he can be pontifex who is himself the pons.—T. F. Torrance, Incarnation: The Person and Life of Christ, 190