Are Atheists Vincibly Ignorant?

In Catholic thought there is what is called vincible ignorance. Here is a definition:

Lack of knowledge for which a person is morally responsible. It is culpable ignorance because it could be cleared up if the person used sufficient diligence. One is said to be simply (but culpably) ignorant if one fails to make enough effort to learn what should be known; guilt then depends on one's lack of effort to clear up the ignorance.

For present purposes, it suffices to say that 'God' refers to the supreme being of the Judeo-Christian tradition.

I hold that there is vincible ignorance on various matters. But I deny that atheists are vincibly ignorant. Some might be, but not qua atheists. Whether or not God exists, one is not morally culpable for denying the existence of God. Nor do I think one is morally culpable if one doubts the existence of God.

If God exists, and one is an atheist, then one is ignorant of God, but it does not follow that one is culpably ignorant.

This puts me at odds with St. Paul, at least on one interpretation of what he is saying at Romans 1: 18-20.