Although Christianity and Judaism are the largest religious groups in the United States, Islam are rapidly catching up in 2040. According to the Pew Institute, the Muslim population will increase to 8.1 million and surpass the Judaism population. The main reason is due to immigration and that the Muslim population is more youthful than the other religious groups.

Link: https://www.cnn.com/2018/01/10/politics/muslim-population-growth-second-religious-group-trnd/index.html