Secularism is not a religion. The Founding Fathers deliberately avoided having a state-sponsored religion. They were men who knew their history. The mixing of government and religion is one of sectarian strife ending with war, torture and cruelty beyond measure. They set up a secular government deliberately.
The evidence of this is Jefferson's letter to the Danbury Baptists, where the words "separation of church and state" originated.
The second piece of evidence is the Treaty of Tripoli, in which Article XI states: "As the Government of the United States of America is not, in any sense, founded on the Christian religion." This treaty was passed by the Senate and signed by John Adams. Treaties have the force of law.
The third is the Constitutional clause that states there will be no religious test for political office.
The First Amendment prohibits the making of any law respecting an establishment of religion, or impeding the free exercise of religion. It frees individuals to seek a higher power in their own way, but just as important, it keeps those who would force their beliefs on the rest of us at bay.
We are one of the most religious nations on earth, yet we are free from the sectarian strife that is so prevalent in the Mideast today and in the 1990s in the Balkans. This can be laid at the feet of our Founding Fathers, who gave us a secular government. One of my favorite bumper stickers: "God, protect me from your followers."