You want me to tell you why God is to be loved and how much. I answer, the reason for loving God is God Himself; and the measure of love due to Him is immeasurable love. Is this plain? Doubtless, to a thoughtful man; but I am debtor to the unwise also. A word to the wise is sufficient; but I must consider simple folk too. Therefore I set myself joyfully to explain more in detail what is meant above.
We are to love God for Himself, because of a twofold reason; nothing is more reasonable, nothing more profitable. When one asks, Why should I love God? he may mean, What is lovely in God? or What shall I gain by loving God? In either case, the same sufficient cause of love exists, namely, God Himself.
And first, of His title to our love. Could any title be greater than this, that He gave Himself for us unworthy wretches? And being God, what better gift could He offer than Himself? Hence, if one seeks for God’s claim upon our love here is the chiefest: Because He first loved us (I John 4.19).
Ought He not to be loved in return, when we think who loved, whom He loved, and how much He loved? For who is He that loved? The same of whom every spirit testifies: Thou art my God: my goods are nothing unto Thee’ (Ps. 16.2, Vulg.). And is not His love that wonderful charity which seeketh not her own’? (I Cor.13.5). But for whom was such unutterable love made manifest? The apostle tells us: When we were enemies, we were reconciled to God by the death of His Son’ (Rom. 5.10). So it was God who loved us, loved us freely, and loved us while yet we were enemies. And how great was this love of His? St. John answers: God so loved the world that He gave His only-begotten Son, that whosoever believeth in Him should not perish, but have everlasting life’ (John 3.16). St. Paul adds: He spared not His own Son, but delivered Him up for us all’ (Rom. 8.32); and the son says of Himself, Greater love hath no man than this, that a man lay down his life for his friends’ (John 15.13).
This is the claim which God the holy, the supreme, the omnipotent, has upon men, defiled and base and weak. Some one may urge that this is true of mankind, but not of angels. True, since for angels it was not needful. He who succored men in their time of need, preserved angels from such need; and even as His love for sinful men wrought wondrously in them so that they should not remain sinful, so that same love which in equal measure He poured out upon angels kept them altogether free from sin. — from On Loving God (c. 1125-30)