Objection 1: It would seem that God is loved out of charity, not for Himself but for the sake of something else. For Gregory says in a homily (In Evang. xi): "The soul learns from the things it knows, to love those it knows not," where by things unknown he means the intelligible and the Divine, and by things known he indicates the objects of the senses. Therefore God is to be loved for the sake of something else.
Obj. 2: Further, love follows knowledge. But God is known through something else, according to Rom. 1:20: "The invisible things of God are clearly seen, being understood by the things that are made." Therefore He is also loved on account of something else and not for Himself.
Obj. 3: Further, "hope begets charity" as a gloss says on Matt. 1:1, and "fear leads to charity," according to Augustine in his commentary on the First Canonical Epistle of John (In prim. canon. Joan. Tract. ix). Now hope looks forward to obtain something from God, while fear shuns something which can be inflicted by God. Therefore it seems that God is to be loved on account of some good we hope for, or some evil to be feared. Therefore He is not to be loved for Himself.
On the contrary, According to Augustine (De Doctr. Christ. i), to enjoy is to cleave to something for its own sake. Now "God is to be enjoyed" as he says in the same book. Therefore God is to be loved for Himself.
I answer that, The preposition "for" denotes a relation of causality. Now there are four kinds of cause, viz., final, formal, efficient, and material, to which a material disposition also is to be reduced, though it is not a cause simply but relatively. According to these four different causes one thing is said to be loved for another. In respect of the final cause, we love medicine, for instance, for health; in respect of the formal cause, we love a man for his virtue, because, to wit, by his virtue he is formally good and therefore lovable; in respect of the efficient cause, we love certain men because, for instance, they are the sons of such and such a father; and in respect of the disposition which is reducible to the genus of a material cause, we speak of loving something for that which disposed us to love it, e.g. we love a man for the favors received from him, although after we have begun to love our friend, we no longer love him for his favors, but for his virtue. Accordingly, as regards the first three ways, we love God, not for anything else, but for Himself. For He is not directed to anything else as to an end, but is Himself the last end of all things; nor does He require to receive any form in order to be good, for His very substance is His goodness, which is itself the exemplar of all other good things; nor again does goodness accrue to Him from aught else, but from Him to all other things. In the fourth way, however, He can be loved for something else, because we are disposed by certain things to advance in His love, for instance, by favors bestowed by Him, by the rewards we hope to receive from Him, or even by the punishments which we are minded to avoid through Him.
Reply Obj. 1: From the things it knows the soul learns to love what it knows not, not as though the things it knows were the reason for its loving things it knows not, through being the formal, final, or efficient cause of this love, but because this knowledge disposes man to love the unknown.
Reply Obj. 2: Knowledge of God is indeed acquired through other things, but after He is known, He is no longer known through them, but through Himself, according to John 4:42: "We now believe, not for thy saying: for we ourselves have heard Him, and know that this is indeed the Saviour of the world."
Reply Obj. 3: Hope and fear lead to charity by way of a certain disposition, as was shown above (Q. 17, A. 8; Q. 19, AA. 4, 7, 10).