I think that's a little simplistic. Dukat was always a very complex character, I think that just saying that he was opportunistic isn't giving him enough credit, just like Sisko saying he was evil is too simplistc. Yes, he was an opportunist, and yes, he did bad things, but by the time he turned to a being a cult leader, the man had a very broken mind. Perhaps he did believe in the pah-wraiths to some extent, if only that they were the only aliens he knew of that could emiminate the Prophets and remove their influence.
I agree that his rivalry with Sisko had a great deal to do with it, as well. He wanted to be perceived as the hero of Bajor, and when Sisko took that place, without having to do anything but show up to the party, he may have felt cheated. Ruling the pah-wraith cult, however small it might have been, was his way of trying to attain the position he felt he deserved.