John Chrysostom
Catena Aurea · Johannesevangelium 3
He that cometh from above is above all: he that is of the earth is earthly, and speaketh of the earth: he that cometh from heaven is above all. And what he hath seen and heard, that he testifieth; He that is of the earth is of the earth; i. e. is earthly, and speaketh of the earth, speaketh earthly things.
(Hom. xxx. 1) But after this high and solemn mention of Christ, his tone lowers: And what he hath seen and heard, that he testifieth. As our senses are our surest channels of knowledge, and teachers are most depended on who have apprehended by sight or hearing what they teach, John adds this argument in favour of Christ, that, what he hath seen and heard, that he testifieth; meaning that every thing which He saith is true. I want, saith John, to hear what things He, Who cometh from above, hath seen and heard, i. e. what He, and He alone, knows with certainty.