4 Hear. “Behold what manner of love the Father has given us, that we should be called sons of God, and be (such).” For whoso are called sons, and are not sons, what profits them the name where the thing is not? How many are called physicians, who know not how to heal! How many are called watchers, who sleep all night long! So, many are called Christians, and yet in deeds are not found such; because they are not this which they are called, that is, in life, in manners, in faith, in hope, in charity.
But what have ye heard here, brethren? “Behold, what manner of love the Father has bestowed upon us, that we should be called, and should be, the sons of God: therefore the world knows us not, because it has not known Him, us also the world knows not.” There is a whole world Christian, and a whole world ungodly; because throughout the whole world there are ungodly, and throughout the whole world there are godly: those know not these. In what sense, think we, do they not know them?
They deride them that live good lives. Mark well and see: for haply there are such also among you. Each one of you who now lives godly, who despises worldly things, who does not choose to go to spectacles, who does not choose to make himself drunken as it were by solemn custom, yea, what is worse, under countenance of holy days to make himself unclean: the man who does not choose to do these things, how is he derided by those who do them! Would he be scoffed at if he were known?
But why is he not known? “The world knows Him not.” Who is “the world”? Those inhabiters of the world. Just as we say, “a house;” meaning, its inhabitants. These things have been said to you again and again, and we forbear to repeat them to your disgust. By this time, when you hear the word “world,” in a bad signification, you know that you must understand it to mean only lovers of the world because through love they inhabit, and by inhabiting have become entitled to the name.
Therefore the world has not known us, because it has not known Him. He walked here Himself, the Lord Jesus Christ in the flesh; He was God, He was latent in weakness. And wherefore was He not known? Because He reproved all sins in men. They, through loving the delights of sins, did not acknowledge the God: through loving that which the fever prompted, they did wrong to the Physician.
Source: Homilies on the First Epistle of John (New Advent)