The Pyromaniacs are on a roll!
When it comes to disagreements between Christians, we must categorize our differences. Baby baptizers cannot meet with adult baptizers (would babies only be baptized every other week? Alternating with teaching about how the Bible describes credo-baptism as the right way?) One church cannot say we should dedicate both Sunday and Saturday as the one day for God.
But, these groups should be agreeable. Certainly not denying that one is not a Christian, simply for disagreeing.
There is an excellent list of Scriptures at the Team Pyro link. I would add Matthew 22:36-38. Jesus did not say all commandments are equally important, although there, it was more about two commandments summarizing the rest.