History shows how people in power, particularly those who wield the power of governments, use religion to control others. I am convinced that James Madison and the founders of the United States were intimately aware of the dangers religion and governments had on each other, which is why they built a border between the two via the First Amendment to the United States Constitution.
>Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances. page 
We tend to think of the first amendment in personal terms. Athiests do not want children to be forced to pray in schools; Christians do not want to be forced to provide birth control (through insurance) or provide services to LGBTQ.
But I think the real purpose of the first amendment lies within the recognition of the power "the name of God" has on people. (See In The Name Of God) In fact, that power is the most likely reason why Constantine made Christianity the "official" religion of the Roman Empire, it was a blatant use of religion to keep the empire in power.
Of course, Constantine was not the first to wield the power of religion to control people. The Old Testament chronicles how governments and religion abuse each other to raise up the powerful and oppress the lowly.
Christians ought to be particularly sensitive to the combustibility of religion and government as the collaboration of the two led to Jesus's Crucifixion. Even more, Jesus's teaching of the Reign of God, or Kingdom of God, is the alternative to the abusive power of religion and government.
Despite Constantine's efforts, the Roman Empire eventually fell way but government's recognition of and use of the power of religion, and vice versa, grew only stronger and lead to massive loss of life. First were the Crusades and then later the wars throughout Europe after the Protestant Reformation.
From Pontius Pilate and Caiaphas to Constantine to Henry VIII you have a history of tyranny at the hands of government and religions abusing each other, and that is the legacy that the Founders knew well when they set out to form a new government.
Unfortunately, what the founders did not anticipate was the shadow government that now exists in the form of political parties. The First Ammendment firewall prevents the United States government to wield the power of the pulpit but it does not prevent political parties from doing the same.
Republicans in particular seized upon religion to win southern states in Presidential elections, particularly when Ronald Reagan claimed an anti-abortion stance to gain evangelical votes.
Sadly, power hungry Christian leaders do not see the wisdom of the First Amendment in terms of the gospel. Power corrupts and you see that in the dis congruence between the means in the teaching of Jesus and the ends of overturning Roe v. Wade. The consequence is a loss of credibility and therefore people unwilling to hear the Good News.