34 By this the holy Apostles did not understand that He had gone forth, in the sense of having been sent, from God. For they had often heard Him confess, in His earlier discourses, that He was sent; but what they hear now is the express statement that He had gone forth from God. This opens their eyes to perceive from His works His Divine nature. The fact that He had gone forth from God makes clear to them His true Divinity, and so they say, Now therefore we are sure that You know all things, and needest not that any man should ask You; by this we believe that You went forth from God. The reason why they believe that He went forth from God is that He both can, and does, perform the works of God.
Their perfect assurance of His Divine nature is the result of their knowledge, not that He has come from God, but that He did go forth from God. Accordingly we find that it is this truth, now heard for the first time, which clenches their faith. The Lord had made two statements; I went forth from God, and I have come from the Father into this world. One of these, I have come from the Father into this world, they had often heard, and it awakens no surprise.
But their reply makes it manifest that they now believe and understand the other, that is, I went forth from God. Their answer, By this we believe that You went forth from God, is a response to it, and to it only; they do not add, 'And has come from the Father into this world.' The one statement is welcomed with a declaration of faith; the other is passed over in silence. The confession was wrung from them by the sudden presentation of a new truth, which convinced their reason and constrained them to avow their certainty.
They knew already that He, like God, could do all things; but His birth, which accounted for that omnipotence, had not been revealed. They knew that He had been sent from God, but they knew not that He had gone forth from God. Now at last, taught by this utterance to understand the ineffable and perfect birth of the Son, they confess that He had spoken to them without a proverb.
Source: On the Trinity (New Advent)