One of the worst arguments, especially for Christians, is the "slippery slope" argument.
It happens in worship. "Oh, if there's instruments (or a specific instrument), that will lead to entertainment-based worship." Church history shows a WIDE latitude in worship practices, and music in particular.
It happens in biblical "literalism". "Oh, if this (non-doctrinal) passage isn't literally true (regarding, for example, the age of the earth), that will lead to a rejection of the historicity of the bible."
It happens in morals. "Oh, if you let yourself become the tiniest bit angry, you'll become a slave to it." The Pharisees especially LOVED creating extra rules to "protect" people from sinning... by burdening them with man-made rules.
It happens with creeds. "Oh, if this person deviates just a bit from the historic confession, it opens the door to heresy." Where's the robust dialogue that typified the Reformation?
The "slippery slope" argument is bad because it shows a lack of faith in your brother or sister in Christ. A lack of faith that the Holy Spirit guides and preserves him or her. A lack of faith that Christ -- the head of his church, the king of his heavenly kingdom -- is the one who protects and fights for the church.
The contrast to this is grace and charity. Grace allows us to overlook offenses, and charity allows us to assume the best of others' intentions. Charity especially allows us to see ALL Christians -- even across lines like Protestant, Roman Catholicism, and Orthodox -- as brothers and sisters. How many of us (myself included) haven't laughed or mocked when trouble or even scandal came to some other denomination or wing of the church? Lord, give us love, unity, and charity.
