God punishes them. God did not "cut off" the Gentiles and leave them with no knowledge at all of him. Melchizedek and Balaam were both priests of God outside of the Law of Moses. There could have been others, although the scripture is silent. The point is that there was enough about God which could be known that the evil men of Romans 1:18 have no excuse for their action. Compare Acts 17:27.

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Old Testament