2 Although God can do by His own power all that is effected by created natures, nevertheless in the counsels of His loving Providence He has preferred to help men by the instrumentality of men. And, as in the natural order He does not usually give full perfection except by means of man's work and action, so also He makes use of human aid for that which lies beyond the limits of nature, that is to say, for the sanctification and salvation of souls. But it is obvious that nothing can be communicated amongst men save by means of external things which the senses can perceive. For this reason the Son of God assumed human nature-"who being in the form of God.... emptied himself, taking the form of a servant, being made in the likeness of man" ( Philipp . ii., 6,7)-and thus living on earth He taught his doctrine and gave His laws, conversing with men.
Source: Satis Cognitum (Vatican.va)