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.”
10. “For all that is in the world, is the lust of the flesh, and the lust of the eyes, and the pride of life,” three things he has said, which are not of the Father, but are of the world. And the world passes away, and the lust thereof: but he that does the will of God abides for ever, even as He abides for ever. Why am I not to love what God made? What will you? Whether will you love the things of time, and pass away with time; or not love the world, and live to eternity with God? The river of temporal things hurries one along: but like a tree sprung up beside the river is our Lord Jesus Christ. He assumed flesh, died, rose again, ascended into heaven. It was His will to plant Himself, in a manner, beside the river of the things of time. Are you rushing down the stream to the headlong deep? Hold fast the tree. Is love of the world whirling you on? Hold fast Christ. For you He became temporal, that you might become eternal; because He also in such sort became temporal, that He remained still eternal. Something was added to Him from time, not anything went from His eternity. But you were born temporal, and by sin wast made temporal: you were made temporal by sin, He was made temporal by mercy in remitting sins. How great the difference, when two are in a prison, between the criminal and him that visits him! For upon a time a person comes to his friend and enters in to visit him, and both seem to be in prison; but they differ by a wide distinction. The one, his cause presses down: the other, humanity has brought there. So in this our mortal state, we were held fast by our guiltiness, He in mercy came down: He entered in unto the captive, a Redeemer not an oppressor. The Lord for us shed His blood, redeemed us, changed our hope. As yet we bear the mortality of the flesh, and take the future immortality upon trust: and on the sea we are tossed by the waves, but we have the anchor of hope already fixed upon the land.
Source: Homilies on the First Epistle of John (New Advent)