What constitutes a "secondary" issue within Christianity is not a difficult thing to determine. I don't think, anyway. There is constant debate about so-called secondary issues - those issues that are not essential to the Christian faith as far as salvation. Recently I have read a number of things about this, and quite a few people seem to think that secondary issues are made primary depending on who it is doing the deciding, and we'll never come to agreement. So, it's a hopeless topic. But, hey folks, I think it is easier than most people think.
The essentials of the Christian faith are not difficult to identify. The virgin birth, the real and actual death, burial and resurrection of Christ. The Lordship of Christ. The gospel. The second coming, and so on. These things have been held throughout history by the majority of the church, and they continue to be core beliefs. The early creeds are an example. Whether somebody drinks beer or wears certain clothes or allows their daughter to go to college or has a certain form of church government or whether women wear pants or even whether somebody sprinkles or dunks during baptism simply aren't essential doctrines. People make them out to be, and that's the problem. Denying that Jesus came in the flesh is a major problem, but denying that every last Christian child must be homeschooled isn't.
And there's a big difference between having convictions that you un-hypocritically hold to and judging other people based on those convictions. The list of trivial things that are used to judge and condemn people - even to an eternity in hell - doesn't end. Think about it. God is going to cast a professing Christian into the lake of fire on judgment day to spend an eternity in hell because they drank a beer now and then? Because they wear the latest fashions and dress well? God is going to let you into heaven but stick you in a golden corner with a jewel laden dunce cap on because you baptized people the wrong way? Think about what that means. We would be willing to wish the worst on our brothers and sisters because of our personal preferences and pet doctrines? What kind of Christian attitude is that? It's the kind of attitude that Jesus says will condemn the Pharisees.
I'm not claiming innocence on this, as I spent a good amount of time in legalistic churches and groups, and learned how to judge people harshly on secondary issues. But once God shows you the ugliness of it all, it is refreshing to not waste so much energy on condemning others. It's a freeing thing. And that's one of the things that Jesus came for.