Having by all these means quenched their passion, he afterwards speaks more openly of Christ; for before this it was useless to utter words which could never have gained a place in the understanding of his hearers: but when he hath pulled up the thorns, he then boldly casts in the seed, saying, "He that cometh from above is above all. And what He hath heard He speaketh, and what He hath seen He testifieth; and no man receiveth His testimony."
Having uttered something great and sublime concerning Him, he again brings down his discourse to a humbler strain. For the expression, "what He hath heard and seen," is suited rather to a mere man. What He knew He knew not from having learned it by sight, or from having heard it, but He included the whole in His Nature, having come forth perfect from the Bosom of His Father, and needing none to teach Him. For, "As the Father," He saith, "knoweth Me, even so know I the Father." (c. x. 13.) What then means, "He speaketh that He hath heard, and testifieth that He hath seen"? Since by these senses we gain correct knowledge of everything, and are deemed worthy of credit when we teach on matters which our eyes have embraced and our ears have taken in, as not in such cases inventing or speaking falsehoods, John desiring here to establish this point, said, "What He hath heard and seen": that is, "nothing that cometh from Him is false, but all is true." Thus we when we are making curious enquiry into anything, often ask, "Didst thou hear it?" "Didst thou see it?" And if this be proved, the testimony is indubitable.
Having uttered something great and sublime concerning Him, he again brings down his discourse to a humbler strain. For the expression, "what He hath heard and seen," is suited rather to a mere man. What He knew He knew not from having learned it by sight, or from having heard it, but He included the whole in His Nature, having come forth perfect from the Bosom of His Father, and needing none to teach Him. For, "As the Father," He saith, "knoweth Me, even so know I the Father." (c. x. 13.) What then means, "He speaketh that He hath heard, and testifieth that He hath seen"? Since by these senses we gain correct knowledge of everything, and are deemed worthy of credit when we teach on matters which our eyes have embraced and our ears have taken in, as not in such cases inventing or speaking falsehoods, John desiring here to establish this point, said, "What He hath heard and seen": that is, "nothing that cometh from Him is false, but all is true." Thus we when we are making curious enquiry into anything, often ask, "Didst thou hear it?" "Didst thou see it?" And if this be proved, the testimony is indubitable.
Source: Homily on the Gospel of John 30