9 “Marvel not, brethren, if the world hate us.” Must one often be telling you what “the world” means? Not the heaven, not the earth, nor these visible works which God made; but lovers of the world. By often saying these things, to some I am burdensome: but I am so far from saying it without a cause, that some may be questioned whether I said it, and they cannot answer. Let then, even by thrusting it upon them, something stick fast in the hearts of them that hear. What is “the world”? The world, when put in a bad sense, is, lovers of the world: the world, when the word is used in praise, is heaven and earth, and the works of God that are in them; whence it is said, “And the world was made by Him.” Also, the world is the fullness of the earth, as John himself has said, “Not only for our sins is He the propitiator, but (for the sins) of the whole world:” he means, “of the world,” of all the faithful scattered throughout the whole earth. But the world in a bad sense, is, lovers of the world. They that love the world, cannot love their brother.
Source: Homilies on the First Epistle of John (New Advent)