In Colonial times, the Anglican Church (the Church of England) had complete control over English society. They had the full support of the Crown behind them, the reigning Monarch being the Supreme Governor of the Church of England. Therefore, the smaller sects of Christianity had to escape to the New World in order to worship freely. The flock of religious settlers led to the colonization of America and the subsequent Revolution.
When the Founding Fathers drafted the Bill of Rights, they sought to ensure that this nation would never commit the authoritarian infringement of rights that England did. That is why we have freedom of speech, freedom of the press, the right to an attorney, the right to a quick and fair trial, and the freedom of religion.
The Freedom of Religion creates the concept of Separation of Church and State: meaning the government can not make any law in the eye of a specific church, create a sect of a church, or make a law that prohibits a certain religion from exercising their beliefs freely.
In recent years, many Evangelical Republicans have used the Freedom of Religion as a reason why governments should not allow Gay Marriage, Abortion, or force vaccines for their kids. The same voters often declare that Islam is a religion of violence and are among the supporters of Trump's ban on Muslim immigrants.
This practice is Sharia Law.
The church governing the people is a Theocracy, which is exactly the model of Sharia Law. Using religion to create laws is the exact opposite of what the Founding Fathers intended. They may have been religious themselves, but they knew that the church and the government needed to remain entirely separate. They also knew that it was the right of every man to worship freely and that connecting the church to the state would make it hard for non-Christians to live freely in this country.
Christians claiming that Gay Marriage infringes upon their Freedom of Religion is ridiculous. It in no way affects them personally and does not change their ability to worship freely. If two men wish to be married, they can, and any church or minister has the absolute right to deny them their services on the basis of religion. That is Freedom of Religion.
For reference on how asinine this frame of thinking is: this same argument could be used during Lent, stating that because one religion can't eat meat on Fridays, no restaurant is allowed to serve any form of meat on Fridays during the long holiday.
That is clearly unfair to all those who are not Catholic or religious, and there is a clear way around it. A Catholic's form of worship is not in jeopardy due to the table next to them enjoying a plate of wings while they believe they should only be eating a fish fry today. And if the restaurant they are at that Friday doesn't offer a meat free option, then that person can easily just go to a place that does. This is Freedom of Religion.
Using religious beliefs to govern is irresponsible and unconstitutional. Look to Jimmy Carter, a Georgian Baptist is considered one of the most devout Presidents, yet he supported Gay Marriage and Abortion from a legal perspective, even though he spiritually disagreed with it. He knew that it was not his duty as President to push his religious beliefs, but to ensure every man, woman, and child, was free to live their life happily and freely. That is the real American Dream.