This past Sunday, my SS class started chapter 2 of Galatians. In the first section, we saw how important it is for us to confront changes to the gospel, while not being confrontational people over non-essential issues.
The Housewife Theologian posted a blog article today that explains very clearly this concept, along with a logical reason why it is both true and important. She cites a lecture by Don Carson on how we should view fellow believers with whom we don't fully agree on all issues regarding our church practices. Carson uses a 'sets' illustration from mathematics to create an image of orthodoxy. It was immensely helpful to me; I wish I'd seen it before the lesson last weekend.
Here's her post.
I highly recommend this blog. You should be reading it regularly. If you only have time for one blog, dump mine and read hers instead. I bet she won't put scary math equations up, there.