Faith is not an opinion, but a certitude. "The substance of things hoped for," says the Apostle--not the phantasies of empty conjecture. You hear the substance. You may not dispute on the faith as you please; you may not wander here and there through the wastes of opinion, the byways of error. By the name, "substance," something certain and fixed is placed before you; you are enclosed between boundaries, you are restrained within unchanging limits.

... St. Bernard of Clairvaux, Treatise on the Errors of Abeilard