9 You have heard that “if any man love the world, the love of the Father is not in him.” Let not any say in his heart that this is false, brethren: God says it; by the Apostle the Holy Ghost has spoken; nothing more true: “If any man love the world, the love of the Father is not in him.” Would you have the Father's love, that you may be joint-heir with the Son? Love not the world. Shut out the evil love of the world, that you may be filled with the love of God. You are a vessel; but as yet you are full.
Pour out what you have, that you may receive what you have not. Certainly, our brethren are now born again of water and of the Spirit: we also some years ago were born again of water and of the Spirit. Good is it for us that we love not the world, lest the sacraments remain in us unto damnation, not as means of strengthening unto salvation. That which strengthens unto salvation is, to have the root of charity, to have the “power of godliness,” not “the form” only. Good is the form, holy the form: but what avails the form, if it hold not the root?
The branch that is cut off, is it not cast into the fire? Have the form, but in the root. But in what way are you rooted so that you be not rooted up? By holding charity, as says the Apostle Paul, “rooted and grounded in charity.” How shall charity be rooted there, amid the overgrown wilderness of the love of the world? Make clear riddance of the woods. A mighty seed you are about to put in: let there not be that in the field which shall choke the seed. These are the uprooting words which he has said: “Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him.”
Source: Homilies on the First Epistle of John (New Advent)