a wealthy hedonistic atheist!

I don’t know if there is just one word that will do the trick. If is has to be just one then I’d chose HEDONIST.

There are a lot of different traits a monk might have. It might be easier to pick one aspect of being a monk and just using that one opposite.

A monk is cloistered. The opposite of that is worldly.

A monk is spiritual. The opposite is profane.

A monk is godly. The opposite of is impious.

A monk is good. The opposite is wicked.

A monk is chaste. The opposite is promiscuous.

A monk is physically poor and spiritually rich. A miser (?) is physically rich and spiritually poor. (There’s probably a better choice, but I can’t help but think of Scrooge.)

Think about what aspect of the monk you want to contrast and work from there.

A nun is pretty much the female equivalent of a monk, so I personally wouldn’t think of that as the OPPOSITE… and I wouldn’t say just “millionaire,” since the definition of a monk is not simply someone who is poor, but rather someone who has taken a vow of chastity, obedience, AND poverty. I’d say that a man-whore exhibits the “opposite” of ALL of those! 🙂