You might have to wait a while longer:  

As an agnostic I am about 3.3% of the US population according to PEW Research Group.   The athiests about 2.4%.

According to a 2012 review by the National Council of Churches, the five largest denominations are 
The Catholic Church, 68,202,492 members
The Southern Baptist Convention, 16,136,044 members
The United Methodist Church, 7,679,850 members
The Church of Jesus Christ of Latter-day Saints, 6,157,238 members
The Church of God in Christ, 5,499,875 members

The Southern Baptist Convention, with over 16 million adherents, is the largest of more than 200 distinctly named Protestant denominations.  In 2007, members of evangelical churches comprised 26% of the American population, while another 18% belonged to mainline Protestant churches, and 7% belonged to historically black churches.