Here is an interesting analysis from the Pew Research Center. 90 percent are Christian.
The new, 116th Congress includes the first two Muslim women ever to serve in the House of Representatives, and is, overall, slightly more religiously diverse than the prior Congress.1
There has been a 3-percentage-point decline in the share of members of Congress who identify as Christian – in the 115th Congress, 91% of members were Christian, while in the 116th, 88% are Christian. There are also four more Jewish members, one additional Muslim and one more Unitarian Universalist in the new Congress – as well as eight more members who decline to state their religious affiliation (or lack thereof).
While the number of self-identified Christians in Congress has ticked down, Christians as a whole – and especially Protestants and Catholics – are still overrepresented in proportion to their share in the general public. Indeed, the religious makeup of the new, 116th Congress is very different from that of the United States population.
Within Protestantism, certain groups are particularly numerous in the new Congress, including Methodists, Anglicans/Episcopalians, Presbyterians and Lutherans. Additionally, Protestants in the “unspecified/other” category make up just 5% of the U.S. public, but 15% of Congress.2 By contrast, some other Protestant groups are underrepresented, including Pentecostals (5% of the U.S. public vs. 0.4% of Congress).