Although the United States is still predominantly populated by Christians, significant changes are taking place in the number of followers of other religious traditions. According to a report from the Association of Statisticians of American Religious Bodies, which does a census of religious groups every ten years, Buddhists represent the second largest religious group generally in the Western States. Judaism is second largest in the Northeast, while Islam holds that position in the Deep South and much of the Midwest. Hindus take second place in Arizona and Delaware, while Bahai's are second in South Carolina.
On the county level, the diversity is much greater. In the Bay Area, Buddhists are the second largest group in San Francisco, San Mateo, and Santa Cruz counties. Judaism is second in Marin County, Islam in Alameda and Contra Costa counties, and Hinduism in Santa Clara county.
Christians make up the majority in all but one county in the country, but it is perhaps misleading to lump all Chrisian groups as one. The largest single Christian group is the Roman Catholic Church, which is the largest group in over 1,200 counties nation-wide. In California as a whole and the Bay Area specifically, Catholic Christians are the most numerous. The second largest group, dominant in nearly as many counties, is the Southern Baptist Convention.