13 As if anticipating that the inquirer would ask next by what plain mark a young disciple, not yet able to distinguish the truth among so many errors, might find the true Church of Christ, since the clear fulfillment of so many predictions compelled him to believe in Christ, the prophet answers this question in what follows, and teaches that the Church of Christ, which he describes prophetically, is conspicuously visible. His words are: "A glorious high throne is our sanctuary." This glorious throne is the Church of which the apostle says: "The temple of God is holy, which temple you are." The Lord also, foreseeing the conspicuousness of the Church as a help to young disciples who might be misled, says, "A city that is set on an hill cannot be hid." Since, then, a glorious high throne is our sanctuary, no attention is to be paid to those who would lead us into sectarianism, saying, "Lo, here is Christ," or "Lo there."
Lo here, lo there, speaks of division; but the true city is on a mountain, and the mountain is that which, as we read in the prophet Daniel, grew from a little stone till it filled the whole earth. And no attention should be paid to those who, professing some hidden mystery confined to a small number, say, Behold, He is in the chamber; behold, in the desert: for a city set on an hill cannot be hid, and a glorious high throne is our sanctuary.
Source: Reply to Faustus the Manichaean (New Advent)