15 “And him that will come to me, I will not cast out. For I came down from heaven, not to do my own will, but the will of Him that sent me.” Is it for that reason that You will not cast out him that shall come unto You, because You have descended from heaven, not to do Your own will, but the will of Him that sent You? Great mystery! I beseech you, let us knock together; something may come forth to us which may feed us, according to that which has delighted us. That great and sweet secret dwelling-place: “He that will come to me.”
Give heed, give heed, and weigh the matter: “He that will come unto me, I will not cast out.” Why? “Because I came down from heaven, not to do my own will, but the will of Him that sent me.” Is it then the very reason why Thou castest not out him that comes unto You, that You came down from heaven, not to do Your own will, but the will of Him that sent You? The very reason. Why do we ask whether it be the same? The same it is; Himself says it. For it would not be right in us to suspect Him to mean other than He says, “Whoso will come to me, I will not cast out.”
And, as if you asked, wherefore? He answered, “Because I came not to do my own will, but the will of Him that sent me.” I am afraid that the reason why the soul went forth away from God is, that it was proud; nay, I do not doubt it. For it is written, “Pride is the beginning of all sin; and the beginning of man's pride is a falling away from God.” It is written, it is firm and sure, it is true. And hence what is said of proud mortal man, clad in the tattered rags of the flesh, weighed down with the weight of a corruptible body, and withal extolling himself, and forgetting with what skin-coat he is clothed—what, I ask, says the Scripture to him?
“Why is dust and ashes proud?” Why proud! Let the Scripture tell why. “Because in his life he put forth his inmost parts.” What is “put forth,” but “threw afar off”? This is to go forth away. For to enter within, is to long after the inmost parts; to put forth the inmost parts, is to go forth away. The proud man puts forth the inmost parts, the humble man earnestly desires the inmost parts. If we are cast out by pride, let us return by humility.
Source: Tractates on the Gospel of John (New Advent)