Accordingly, if Celsus were to ask us how we think we know God, and how we shall be saved by Him, we would answer that the Word of God, which entered into those who seek Him, or who accept Him when He appears, is able to make known and to reveal the Father, who was not seen (by any one) before the appearance of the Word. And who else is able to save and conduct the soul of man to the God of all things, save God the Word, who, “being in the beginning with God,” became flesh for the sake of those who had cleaved to the flesh, and had become as flesh, that He might be received by those who could not behold Him, inasmuch as He was the Word, and was with God, and was God?
And discoursing in human form, and announcing Himself as flesh, He calls to Himself those who are flesh, that He may in the first place cause them to be transformed according to the Word that was made flesh, and afterwards may lead them upwards to behold Him as He was before He became flesh; so that they, receiving the benefit, and ascending from their great introduction to Him, which was according to the flesh, say, “Even if we have known Christ after the flesh, yet henceforth know we Him no more.” Therefore He became flesh, and having become flesh, “He tabernacled among us,” not dwelling without us; and after tabernacling and dwelling within us, He did not continue in the form in which He first presented Himself, but caused us to ascend to the lofty mountain of His word, and showed us His own glorious form, and the splendour of His garments; and not His own form alone, but that also of the spiritual law, which is Moses, seen in glory along with Jesus.
He showed to us, moreover, all prophecy, which did not perish even after His incarnation, but was received up into heaven, and whose symbol was Elijah. And he who beheld these things could say, “We beheld His glory, the glory as of the only-begotten of the Father, full of grace and truth.” Celsus, then, has exhibited considerable ignorance in the imaginary answer to his question which he puts into our mouth, “How we think we can know God? And how we know we shall be saved by Him?” for our answer is what we have just stated.
Source: Against Celsus (New Advent)