Spilt ink and wasted breath
When I was a young believer I spent (no, wasted) a great deal of time and energy arguing over issues of no significance. They seemed so important at the time, but with the hindsight and wisdom of older years, I now see them as secondary issues at best, tertiary at worse. They come into the category of Paul's "myths and endless geneaologies" (1 Timothy 1:3)
Men in particular get caught up in this stuff. "Command certain men" says Paul, not to devote themselves to these things which only promote controversy. Oh the needless arguments and heated discussions with fellow brothers and sisters in Christ! Provided regret is channeled into wisdom.....
So how do we decide whether an issue is primary - and worth debating and defending - or secondary?
Here are four 'rules of thumb' which I have found helpful for determining primary doctrines.
(1) How often is the issue mentioned in Scripture?
If a truth is mentioned many times, you know it is a vital doctrine. Creation, sin, salvation, and so on are vital doctrines because they are repeated so many times. But sisters wearing hats in church? Mentioned once and even there interpreters differ over the meaning? Not worth arguing over that one - though you would be surprised how often controversy has been caused by this one issue.
(2) Is the doctrine actually said to be important?
In other words, does Scripture actually say "this is of first importance"? Paul in 1 Corinthians 15 says that doctrines such as the death and resurrection of Christ are of first importance; the complete list is quite small. And when Paul lists the truths we all have in common in Ephesians 4, again its quite a short list.
(3) Is the doctrine required for salvation?
Some doctrines are said to be of such Gospel importance that if you don't believe them you are lost: they are "Gospel doctrines". So the resurrection of Jesus Christ (1 Corinthians 15:14) and the real humanity of Jesus (1 John 4:2) are both said to be required for salvation.
(4) How has the church historically ranked this doctrine?
We all have cultural blindspots. For example, because we live in a scientific era, some Christians can easily make one particular interpretation of the Genesis account the test of orthodoxy - and they don't realise that previous generations of Christians didn't rank their interpretation as important as they did. How important did our brothers and sisters in other ages rank this doctrine?
There are many secondary doctrines, such as exactly how Jesus will return (the details), what about the Jewish nation?, should you be baptised by full immersion, shall we celebrate communion every time we meet or not, what translation shall we use, and so on.
It's good that we know where we stand on these issues; but they do not determine our salvation and they are not worth dividing about - and certainly not worth arguing about or falling out over.