According to this article there seem to be quite a difference between religion of Congress members and that of general population:
The religious makeup of the legislative body is overwhelmingly and disproportionately Christian, according to Pew’s “Faith on the Hill” survey. Nearly 91 percent of members of Congress reported their religious affiliation as Christian, while 71 percent of U.S. adults identify as Christian.
A particular case is that of those with no religious affiliation:
Just one member of Congress, Rep. Kyrsten Sinema (D-Ariz.), identifies as religiously “unaffiliated.” She’s been the only “unaffiliated” member of Congress since she was elected in 2012, even as the nation’s ranks of religious “nones” — those who don’t subscribe to a particular religious creed or simply don’t believe in God — have grown to include nearly one-quarter of U.S. adults.
Question: Why is there a significant difference between religious makeup of the legislative body and that of adult population in US?