The most under-represented is “no affiliation" (only 1 congress-person is listed as “none"). So that is a small fraction of 1%, when compared to the population of persons in the US who are unaffiliated with any religion, which is close to 20%.
And we have a tie, the Native Americans, those who were first here and who were the LAST minority to obtain the vote (1924), exactly 1. Brutal irony.
http://www.huffingtonpost.com/bill-lucey/113th-congress-by-the-num_b_2737382.html
Cap't Jack
What if you are a Native American who is Atheist? Ouch....
I won't be around for it, but wouldn't it be cool, if we, someday, elected an atheist Native American to be POTUS?