Madison originally wasn't for a bill of rights as he was afraid it would lead to the government using it as a list of rights and deny people of other rights not listed . Hence the Ninth Amendment making it clear there are rights not listed in the Constitution.  He had opponents trying to block him from getting votes needed to represent Virginia in ratifying the Constitution.  He was forced to visit special groups to win their vote. One was the Baptists in Virginia. The state church was the Anglican church. Virginia imposed a tax for the church and forced people to take their babies to be baptized by the Anglican priests or suffer being flogged in the public square. Madison met with John Leeland, the most prominent Baptist minister. He gave Leeland his word he would try and do something. This lead to the the 1st Amendment we know today.