Objection 1. It would seem that the object of faith is something seen. For Our Lord said to Thomas (John 20:29): "Because thou hast seen Me, Thomas, thou hast believed." Therefore vision and faith regard the same object.
Objection 2. Further, the Apostle, while speaking of the knowledge of faith, says (1 Corinthians 13:12): "We see now through a glass in a dark manner." Therefore what is believed is seen.
Objection 3. Further, faith is a spiritual light. Now something is seen under every light. Therefore faith is of things seen.
Objection 4. Further, "Every sense is a kind of sight," as Augustine states (De Verb. Domini, Serm. xxxiii). But faith is of things heard, according to Romans 10:17: "Faith... cometh by hearing." Therefore faith is of things seen.
On the contrary, The Apostle says (Hebrews 11:1) that "faith is the evidence of things that appear not."
I answer that, Faith implies assent of the intellect to that which is believed. Now the intellect assents to a thing in two ways. First, through being moved to assent by its very object, which is known either by itself (as in the case of first principles, which are held by the habit of understanding), or through something else already known (as in the case of conclusions which are held by the habit of science). Secondly the intellect assents to something, not through being sufficiently moved to this assent by its proper object, but through an act of choice, whereby it turns voluntarily to one side rather than to the other: and if this be accompanied by doubt or fear of the opposite side, there will be opinion, while, if there be certainty and no fear of the other side, there will be faith.
Now those things are said to be seen which, of themselves, move the intellect or the senses to knowledge of them. Wherefore it is evident that neither faith nor opinion can be of things seen either by the senses or by the intellect.
Reply to Objection 1. Thomas "saw one thing, and believed another" [St. Gregory: Hom. xxvi in Evang.]: he saw the Man, and believing Him to be God, he made profession of his faith, saying: "My Lord and my God."
Reply to Objection 2. Those things which come under faith can be considered in two ways. First, in particular; and thus they cannot be seen and believed at the same time, as shown above. Secondly, in general, that is, under the common aspect of credibility; and in this way they are seen by the believer. For he would not believe unless, on the evidence of signs, or of something similar, he saw that they ought to be believed.
Reply to Objection 3. The light of faith makes us see what we believe. For just as, by the habits of the other virtues, man sees what is becoming to him in respect of that habit, so, by the habit of faith, the human mind is directed to assent to such things as are becoming to a right faith, and not to assent to others.
Reply to Objection 4. Hearing is of words signifying what is of faith, but not of the things themselves that are believed; hence it does not follow that these things are seen.
Source: Summa Theologica (New Advent)