The overwhelming majority were protestant, yes, but not exclusively. Even so, the only reason many of them came here in the first place was to escape the tyranny of other protestant religions, particularly in Britain. Charles Carroll, RC, signed the Declaration, but more importantly, his state, Maryland, under Catholic Lord Baltimore & family, because of the persecution of Catholics in Britain, became the first to grant complete religious freedom. Rhode Island followed suit two years later (when its founder fled puritanism in Massachusetts). And, despite the fact that they were influenced by Catholic and Protestant writers alike, the founders out-thought them all, producing a system that exceeded, exponentially, the sum of its parts.
Yeah America was pretty close to a 100% Protestant creation about as close as a thing could be to it.