4 Wherefore both Moses bore witness to Christ, and John bore witness to Christ, and all the other prophets and apostles bore witness to Christ. Before all these testimonies He places the testimony of His own works. Because through those men too, it was God and none other that bore witness to His Son. But yet in another way God bears testimony to His Son. God reveals His Son through the Son Himself, He reveals Himself through the Son. To Him, if a man shall have been able to reach, he shall need no lamps; and by truly digging deep, he will carry down his building to the rock.
Source: Tractates on the Gospel of John (New Advent)