3 Now this is what follows: and John says, “This my joy therefore is fulfilled.” What is his joy? To rejoice at the Bridegroom's voice. It is fulfilled in me, I have my grace; more I do not assume to myself, lest also I lose what I have received. What is this joy? “With joy rejoices for the Bridegroom's voice.” A man may understand, then, that he ought not to rejoice of his own wisdom, but of the wisdom which he has received from God. Let him ask nothing more, and he loses not what he found.
For many, in that they affirmed themselves to be wise, became fools. The apostle convicts them, and says of them, “Because that which is known of God is manifest to them; for God has showed it unto them.” Hear ye what he says of certain unthankful, ungodly men: “For the invisible things of Him from the creation of the world are seen, being understood by the things that are made, His eternal power likewise, and Godhead; so that they are without excuse.” Why without excuse? “Because, knowing God” (he said not, “because they knew Him not”), “they glorified Him not as God, nor were thankful; but became vain in their imaginations, and their foolish heart was darkened: professing themselves to be wise, they became fools.” If they had known God, they had known at the same time that God, and none other, had made them wise; and they would not then attribute to themselves that which they did not have from themselves, but to Him from whom they had received it.
But by their unthankfulness they became fools. Therefore, what God gave freely, He took from the unthankful. John would not be this; he would be thankful: he confessed to have received, and declared that he rejoiced for the Bridegroom's voice, saying, “Therefore this my joy is fulfilled.”
Source: Tractates on the Gospel of John (New Advent)