4 “The heavens,” it is said, “declare the glory of God.” When this Psalm is read, it occurs to one that not so much the material elements as the heavenly merits seem to offer praise worthy of God. And by the chance of this day's lessons it is made clear what “heavens” declare the glory of God. Look at the holy relics at my right hand and at my left, see men of heavenly conversation, behold the trophies of a heavenly mind. These are the heavens which declare the glory of God, these are His handiwork which the firmament proclaims. For not worldly enticements, but the grace of the divine working, raised them to the firmament of the most sacred Passion, and long before by the testimony of their character and virtues bore witness of them, that they continued steadfast against the dangers of this world.
Source: Letters (New Advent)