8 Has falsehood indeed found something to say, and should not truth find its fitting reply? Let us hear what they say: let us hear what they hear. “We have one Father,” they say, “even God. Then said Jesus unto them, If God were your Father, you would [doubtless] love me; for I proceeded forth and came from God; neither came I of myself, but He sent me.” You call God Father; recognize me, then, as at least a brother. At the same time He gave a stimulus to the hearts of the intelligent, by touching on that which He has a habit of saying, “I came not of myself: He sent me. I proceeded forth and came from God.” Remember what we are wont to say: From Him He came; and from whom He came, with Him He came. The sending of Christ, therefore, is His incarnation. But as respects the proceeding forth of the Word from God, it is an eternal procession. Time holds not Him by whom time was created. Let no one be saying in his heart, Before the Word was, how did God exist? Never say, Before the Word of God was. God was never without the Word, because the Word is abiding, not transient; God, not a sound; by whom the heaven and earth were made, and which passed not away with those things that were made upon the earth. From Him, then, He proceeded forth as God, the equal, the only Son, the Word of the Father; and came to us, for the Word was made flesh that He might dwell among us. His coming indicates His humanity; His abiding, His divinity. It is His Godhead towards which, His humanity whereby, we make progress. Had He not become that whereby we might advance, we should never attain to Him who abides ever.
9. “Why,” He says, “do ye not understand my speech? Even because ye cannot hear my word.” And so they could not understand, because they could not hear. And whence could they not hear, but just because they refused to be set right by believing? And why so? “You are of your father the devil.” How long do ye keep speaking of a father? How often will you change your fathers—at one time Abraham, at another God? Hear from the Son of God whose children ye be: “You are of your father the devil.”
Source: Tractates on the Gospel of John (New Advent)