1 We have heard the words of the holy Gospel; and this that the Lord Jesus says, “If I bear witness of Myself, My witness is not true,” may perplex some. How then is not the witness of the Truth true? Is it not Himself who has said, “I am the Way, and the Truth, and the Life”? Whom then are we to believe, if we must not believe the Truth? For of a surety he is minded to believe nothing but falsehood, who does not choose to believe the truth. So then this was spoken on their principles, that you should understand it thus, and gather this meaning from these words; “If I bear witness of Myself, My witness is not true,” that is, as you think. For He knew well that His Own witness of Himself was true; but for the sake of the weak, and hard of belief, and without understanding, the Sun looked out for lamps. For their weakness of sight could not bear the dazzling brightness of the Sun.
Source: Sermons on Selected Lessons of the New Testament (New Advent)