Let's compare basic theistic belief and faith strictly defined, i.e. explicit propositional faith in response to divine Revelation, in two respects:

A. the way by which the intellect reaches the truth to be assented to,

B. the basis of the intellect's assent to that truth: either because the intellect "sees" it as evident, or because the will commands the intellect to assent to it.

In faith strictly defined:

f.A. The intellect arrives at the truth to be assented to by listening to divine Revelation.

f.B. The will commands the intellect to assent to that truth on the basis of the authority of God who revealed it, Who can neither deceive nor be deceived.

In faith, the will is aided by the grace of God, as stated by St. Thomas: "believing is an act of the intellect assenting to the divine truth by command of the will moved by God through grace" (ST II-II, q. 2, a. 9, resp., quoted by CCC #155) and by the Ecumenical Council Vatican I in its Dogmatic Constitution "Dei Filius", ch. 3 "On faith": "The Catholic Church professes that this faith, which is the beginning of human salvation, is a supernatural virtue, by means of which, with the inspiration and assistance of the grace of God, we believe that the things revealed by Him are true, not because the intrinsic truth of the things has been perceived by the natural light of reason, but because of the authority of God Himself who reveals them, who can neither deceive nor be deceived."

In coming to hold basic theistic belief:

t.A. The intellect arrives at the truth to be assented to by reasoning metaphysically from created things.

t.B. There are two possibilities for the assent of the intellect to that truth:

t.B.a. The intellect may be moved to assent to it by its very object, which is the case when it "sees" as self-evident the Principle of Sufficient Reason (PSR), i.e. the presupposition that reality is ultimately rationally explainable, so that rational demonstrations are reliable even when dealing with metaphysical issues like the ultimate cause of the universe.

t.B.b. The intellect may not "see" PSR as a self-evident first principle, but has to assent to it through an act of choice. In this case, the will commands the intellect to assent to PSR, and thus indirectly to the truths derived on its basis by reasoning metaphysically from created things.

For this case, I propose as hypothesis that the will needs to be assisted by the grace of God just as in the case of faith. I.e., I propose that "the inspiration and assistance of the grace of God" may be required by a significant subset of people for holding PSR, or equivalently that reality is ultimately rationally explainable. In this case, and in this sense only, holding the basic theistic belief may be a kind of faith.

Now, if in the case of faith strictly defined the will commands the intellect to assent to revealed truth on the basis of the authority of God, on what basis does the will command the intellect to assent to PSR in case t.B.b? St. Thomas answers this in ST II-II q. 5 a. 2 resp.:

"As stated above (Question [1], Article [4]; Question [2], Article [1]), the believer's intellect assents to that which he believes, not because he sees it either in itself, or by resolving it to first self-evident principles, but because his will commands his intellect to assent. Now, that the will moves the intellect to assent, may be due to two causes. First, through the will being directed to the good, and in this way, to believe is a praiseworthy action. Secondly, because the intellect is convinced that it ought to believe what is said, though that conviction is not based on objective evidence. Thus if a prophet, while preaching the word of God, were to foretell something, and were to give a sign, by raising a dead person to life, the intellect of a witness would be convinced so as to recognize clearly that God, Who lieth not, was speaking, although the thing itself foretold would not be evident in itself, and consequently the essence of faith would not be removed."

So, in the case of a person who assents to PSR "not because he sees it either in itself, or by resolving it to first self-evident principles" (which is impossible in this case, since PSR is itself a principle), "but because his will commands his intellect to assent", is it the case that, when the will moves the intellect to assent to PSR, it is "through the will being directed to the good"? I posit that it clearly is, which can be shown by noting that the choices are on the one hand everlasting life in the hands of an infinite Reason Who loves each of us, and on the other hand brief life at the mercy of nature's blind forces. In more words, either Ultimate Reality is creative Reason, Who creates each human being in its image, endowing him or her with a spiritual rational soul which subsists after death, or reality does not have an ultimate rational explanation and human beings are just walking-talking apes living a finite life with no meaning, either individually or collectively, since it is well known that the evolution of the sun will end all life on Earth within at most 2 billion years. And in even more words, now Benedict XVI's:

"Benedict XVI" wrote:

In the end, to reach the definitive question I would say: God exists or he does not exist. There are only two options. Either one recognizes the priority of reason, of creative Reason that is at the beginning of all things and is the principle of all things - the priority of reason is also the priority of freedom -, or one holds the priority of the irrational, inasmuch as everything that functions on our earth and in our lives would be only accidental, marginal, an irrational result - reason would be a product of irrationality.

One cannot ultimately "prove" either project, but the great option of Christianity is the option for rationality and for the priority of reason. This seems to me to be an excellent option, which shows us that behind everything is a great Intelligence to which we can entrust ourselves.

http://w2.vatican.va/content/benedict-xvi/en/speeches/2006/april/documents/hf_ben-xvi_spe_20060406_xxi-wyd.html