So because most of the world now uses the Gregorian calendar, therefore most of the world is Catholic? No, we're not. The fact is that the Gregorian calendar became a standard in spite of most people NOT being Catholic.
The fact that "most" of the colonists were Christians of various sorts does not change the fact that Christianity and the Bible had absolutely zero examples to offer as models of a representative republic in which the rights of individual citizens' lives, liberties, and property were protected by law.
Most of the European Christians who fled to the New World in search of "religious freedom" were fleeing from ... other Christians.
The fact is that our founders had observed that every government with an established official religion had led to religious tyranny, and they wisely circumvented such in our First Amendment.
Christians did not want to be forced by the federal goverment to attend or not attend the church of their choice.
The Bible was quoted at the founding convention far more than any other source and was considered of infinite value for forming this nation.