come to the dark side, we have coffee.
I take cream in mine. That puts me more on the dark-brown side.
The original motivation for "separation" was compulsory contributions to a state-supported church, which were prevalent in the colonies. For instance, there was a time in colonial Virginia when everyone in the colony was officially a member of the Church of England and were taxed to support the church. Some people took exception to this. Read Thomas Jefferson's preamble to the Virginia Statute on Religious Freedom (the original version is better than the one that passed) to get a good look at his thoughts on religious freedom. It's a little surprising how modern his thinking was.