35 But the novel sound of these words disturbed the Apostle Philip. A Man is before their eyes; this Man avows Himself the Son of God, and declares that when they have known Him they will know the Father. He tells them that they have seen the Father, and that, because they have seen Him, they shall know Him hereafter. This truth is too broad for the grasp of weak humanity; their faith fails in the presence of these paradoxes. Christ says that the Father has been seen already and shall now be known; and this, although sight, is knowledge.
He says that if the Son has been known, the Father has been known also; and this though the Son has imparted knowledge of Himself through the bodily senses of sight and sound, while the Father's nature, different altogether from that of the visible Man, which they know, could not be learned from their knowledge of the nature of Him Whom they have seen. He has also often borne witness that no man has seen the Father. And so Philip broke forth, with the loyalty and confidence of an Apostle, with the request, Lord, show us the Father, and it suffices us.
He was not tampering with the faith; it was but a mistake made in ignorance. For the Lord had said that the Father had been seen already and henceforth should be known; but the Apostle had not understood that He had been seen. Accordingly he did not deny that the Father had been seen, but asked to see Him. He did not ask that the Father should be unveiled to his bodily gaze, but that he might have such an indication as should enlighten him concerning the Father Who had been seen.
For he had seen the Son under the aspect of Man, but cannot understand how he could thereby have seen the Father. His adding, And it suffices us, to the prayer, Lord, show us the Father, reveals clearly that it was a mental, not a bodily vision of the Father which he desired. He did not refuse faith to the Lord's words, but asked for such enlightenment to his mind as should enable him to believe; for the fact that the Lord had spoken was conclusive evidence to the Apostle that faith was his duty. The consideration which moved him to ask that the Father might be shown, was that the Son had said that He had been seen, and should be known because He had been seen. There was no presumption in this prayer that He, Who had already been seen, should now be made manifest.
Source: On the Trinity (New Advent)