The greatest disparity between the religious makeup of Congress and the people it represents is in the percentage of the unaffiliated — those who describe their religion as atheist, agnostic or “nothing in particular.” Only six members of the 112th Congress (about 1%) do not specify a religious affiliation and none say they are unaffiliated. By contrast, about one-sixth (16%) of U.S. adults are not affiliated with any particular faith.
This group has also been growing very strongly - but the public political culture closets it.
I'm not sure that this is really an example of "closeting" unless one assumes that there are members of Congress who are "unaffiliated" (I really dislike that term, BTW) in their hearts but profess otherwise for any variety of reasons (family, social pressure, or their political careers). I would guess that this is the case for a handful, but probably not many.
But if we take the members' responses at face value, this seems to me a simple function of majoritarian politics. Religious minority groups that are represented at all among the members tend to be somewhat geographically concentrated, meaning that while they are minorities nationally, they may be less so locally. This is clearly true for Mormons (Utah), and probably to some extent for Orthodox and Jewish people as well (urban districts.)
Atheists are pretty well-distributed geographically. I don't think most of the other 83.9% of the American public are consciously against atheists holding office...but the minority of them that do care, care about it a LOT. And with numbers that disparate, that's really all it takes to keep professing atheists out of the halls of congress, for now.