Now it is one thing to say: "I believe God" (credere Deum), for this indicates the object. It is another thing to say: "I believe God" (credere Deo), for this indicates the one who testifies. And it is still another thing to say: "I believe in God" (in Deum), for this indicates the end. Thus God can be regarded as the object of faith, as the one who testifies, and as the end, but in different ways. For the object of faith can be a creature, as when I believe in the creation of the heavens. Again, a creature can be one who testifies, for I believe Paul (credo Paulo) or any of the saints. But only God can be the end of faith, for our mind is directed to God alone as its end. Now the end, since it has the character of a good, is the object of love. Thus to believe in God (in Deum) as in an end is proper to faith living through the love of charity.
Thomas Aquinas, Commentary on the Gospel of John: Chapters 6-12, 901, The Catholic University of America Press (Washington, DC: 2010) p. 21