1/4, that's definitely a huge group, I thought it's like 10% and even less.
Roman Catholics are the largest Christian Denomination in the United States; its the 4th largest Catholic nation in the world.
The United States' population is about 73% Christian. Of the some 73% Christians, about half of that belong to Protestant denominations such as:
United Methodist Church
Presbyterian Church
Anglican Church
Christian Orthodoxy
Baptist Churches
Evangelical Churches etc