John Calvin Commentary Romans 1:19

"because that which is known of God is manifest in them; for God manifested it unto them." — Romans 1:19 (ASV)

Inasmuch as what may be known of God, etc. He thus designates what we ought to know of God; and he means all that pertains to the setting forth of the glory of the Lord, or, which is the same thing, whatever ought to move and excite us to glorify God.

And by this expression he intimates, that God in His greatness can by no means be fully comprehended by us, and that there are certain limits within which people ought to confine themselves, since God accommodates to our small capacities what He testifies of Himself. Insane, therefore, are all those who seek to know by themselves what God is: for the Spirit, the teacher of perfect wisdom, does not in vain invite our attention to what may be known, τὸ γνωστὸν; and by what means this is known, he immediately explains.

And he said, in them rather than to them, for the sake of greater emphasis. For though the Apostle frequently adopts Hebrew phrases, and ב, beth, is often redundant in that language, yet here he seems to have intended to indicate a manifestation by which they might be so constrained that they could not evade it. For each of us undoubtedly finds it engraved on his own heart. In saying that God has made it manifest, he means that human beings were created to be spectators of this formed world, and that eyes were given to them so that, by looking at such a beautiful picture, they might be led up to the Author Himself.