A recent study (the 2012 Religious Congregations and Membership Study) shows that in over half of the United States, The Church of Jesus Christ of Latter-day Saints is the fastest-growing religion. The Muslim faith is also growing rapidly in comparison to other faiths. .

The study, conducted every 10 years, tracks religious growth across the United States at the county level.

One Huffington Post article explains more about the survey:

Overall, the survey identified nearly 350,000 religious congregations in the United States, from Albanian Orthodox to Zoroastrian. Those churches, temples and mosques are the spiritual home for 150.6 million Americans, and researchers say they were able to capture 90 percent of all U.S. congregations.

Another Huffington Post article explains that

According to the report, Mormons are the fastest-growing religious group in 26 states and have reported 2 million new adherents in 295 counties over the past decade. Muslims have gained 1 million new adherents in 197 counties — up from 1 million in 2000 to 2.6 million in 2010. (-from this article)

In a recent infographic, the Newsroom of The Church of Jesus Christ of Latter-day Saints noted that Mormonism is the fourth largest Christian religion in the United States, but even with the growth within the United States, more than half of the 14 million Mormons worldwide live outside of the U.S.