Romans 1:19

The Apostle goes on to show how the Gentiles came to have such a knowledge of right, and how they repressed and contravened it.
They had it, because all the knowledge that mankind generally possessed of God they also possessed. So much as could be known without special revelation they knew.

That which may be known. — Rather, that which is (generally and universally) known — the truths of so-called “natural religion.”

Is manifest in them. — Manifest or imprinted upon their consciences, because God had so imprinted it upon them. The marginal rendering, “to them,” is hardly tenable.