Objection 1. It would seem that the act of faith is unsuitably distinguished as believing God, believing in a God, and believing in God. For one habit has but one act. Now faith is one habit since it is one virtue. Therefore it is unreasonable to say that there are three acts of faith.
Objection 2. Further, that which is common to all acts of faith should not be reckoned as a particular kind of act of faith. Now "to believe God" is common to all acts of faith, since faith is founded on the First Truth. Therefore it seems unreasonable to distinguish it from certain other acts of faith.
Objection 3. Further, that which can be said of unbelievers, cannot be called an act of faith. Now unbelievers can be said to believe in a God. Therefore it should not be reckoned an act of faith.
Objection 4. Further, movement towards the end belongs to the will, whose object is the good and the end. Now to believe is an act, not of the will, but of the intellect. Therefore "to believe in God," which implies movement towards an end, should not be reckoned as a species of that act.
On the contrary is the authority of Augustine who makes this distinction (De Verb. Dom., Serm. lxi--Tract. xxix in Joan.).
I answer that, The act of any power or habit depends on the relation of that power or habit to its object. Now the object of faith can be considered in three ways. For, since "to believe" is an act of the intellect, in so far as the will moves it to assent, as stated above (1, ad 3), the object of faith can be considered either on the part of the intellect, or on the part of the will that moves the intellect.
If it be considered on the part of the intellect, then two things can be observed in the object of faith, as stated above (Question 1, Article 1). One of these is the material object of faith, and in this way an act of faith is "to believe in a God"; because, as stated above (Question 1, Article 1) nothing is proposed to our belief, except in as much as it is referred to God. The other is the formal aspect of the object, for it is the medium on account of which we assent to such and such a point of faith; and thus an act of faith is "to believe God," since, as stated above (Question 1, Article 1) the formal object of faith is the First Truth, to Which man gives his adhesion, so as to assent to Its sake to whatever he believes.
Thirdly, if the object of faith be considered in so far as the intellect is moved by the will, an act of faith is "to believe in God." For the First Truth is referred to the will, through having the aspect of an end.
Reply to Objection 1. These three do not denote different acts of faith, but one and the same act having different relations to the object of faith.
This suffices for the Reply to the Second Objection.
Reply to Objection 3. Unbelievers cannot be said "to believe in a God" as we understand it in relation to the act of faith. For they do not believe that God exists under the conditions that faith determines; hence they do not truly imply believe in a God, since, as the Philosopher observes (Metaph. ix, text. 22) "to know simple things defectively is not to know them at all."
Reply to Objection 4. As stated above (I-II, 9, 1) the will moves the intellect and the other powers of the soul to the end: and in this respect an act of faith is "to believe in God."
Source: Summa Theologica (New Advent)