Other expressions for "to be the devil's advocate"?

You could say you are doing something for the sake of argument.

But, really, "devil's advocate" is a familiar phrase that few people take exception to. In fact, it's even used by religious people. For example,

During the canonization process of the Roman Catholic Church, the Promoter of the Faith (Latin: promotor fidei), popularly known as the Devil's advocate (Latin: advocatus diaboli), was a canon lawyer appointed by Church authorities to argue against the canonization of the candidate.

If those religious worthies may use the term, I don't see anything preventing you from doing likewise.

I prefer @Robusto's response, but if you're still uncomfortable with the term, consider "opposition", "opposer", or "contrarian".