Via the Washington Post:
In the Western U.S., Buddhists represent the largest non-Christian religious bloc in most states. In 20 states, mostly in the Midwest and South, Islam is the largest non-Christian faith tradition. And in 15 states, mostly in the Northeast, Judaism has the most followers after Christianity. Hindus come in second place in Arizona and Delaware, and there are more practitioners of the Baha’i faith in South Carolina than anyone else. All these data come from the Association of Statisticians of American Religious Bodies, which conducts a U.S. Religion Census every 10 years.
Hit the link for a breakdown by every US county. Christianity dominates in all but one of the 3143 counties in the nation.