During the procession of the Black Nazarene a few days ago, I saw that this quote from Thomas Aquinas got repeatedly posted.
To one who has faith, no explanation is necessary.
To one without faith, no explanation is possible.They apparently use the quote to make it appear that faith and reason are opposed to each other. It is to justify the apparent fanaticism of the devotees of the Black Nazarene.
But was that what Aquinas meant in that quote? Dr. Norman Geisler, an expert on Thomist philosophy, disagreed that Aquinas taught that faith and reason are mutually exclusive. He summarized his view on the relationship of faith and reason this way:
[Aquinas] stresses the need for reason both before, during, and after believing. Even the mysteries of faith are not irrational. But true faith in God comes only by the grace of God. Indeed, he believes that faith can never be based on reason. At best it can only be supported by reason. Thus, reason and evidence are never coercive of faith. … For reason can be used to demonstrate that God exists, but it can never in itself persuade someone to believe in God. Only God can do this, working along with the evidence in and through their free choice. In other words, according to Aquinas, “reason accompanies faith, but it does not cause faith.”  Reason has its limits. But that doesn’t mean it’s no longer necessary when one believes. If I may add, while it does not go against reason, faith goes beyond reason.
It is true that faith is a gift of God (Ephesians 2:8-9). According to Romans 10:17, “So faith comes from hearing, and hearing through the word of Christ.” (ESV) But it is also true that faith has to have basis. According to 1 Corinthians 15:1-2,
Now I would remind you, brothers, of the gospel I preached to you, which you received, in which you stand, and by which you are being saved, if you hold fast to the word I preached to you—unless you believed in vain. (Emphasis added)In the New Living Translation, that last clause goes this way, “unless, of course, you believed something that was never true in the first place.”
In short, you must have a reason to believe. Faith, after all, is not blind. If it’s blind, it’s not faith at all.
Brothers and sisters, our faith must be reasonable or it’s not faith at all.
 Norman L. Geisler (9 December 2014), “The Apologetics of Thomas Aquinas,” Apologetics Resource Center, retrieved from https://arcapologetics.org/.