Paul speaks to the Romans about a 'certain type' of person. The type who knew God, but who wouldn't worship him as God or even give him thanks. And they began to think up foolish ideas of what God was like. The result was that their minds became dark and confused.
So who are these people?
They are those who have recognized, even through creation, the beauty of nature that there IS a Creator. However, they have chosen not to honour Him, or thank Him for what He has done. Instead, in pride, they make up their own ideas of what 'god' is like - making Him vague, or human or animal, disinterested or cruel. The result of their choice to worship their own god-image rather than the Living God is that their minds become dark. And confused. They claim to be wise, but they became utter fools instead, muttering incantations over jars of catguts or stones. Connecting with spiritual powers they do not understand, neither of which are they in control of as they assume they are. They worship idols made to look like mere people, or birds and animals and snakes instead of Yahweh. So He lets them go ahead and do whatever shameful things their hearts desire.