This excerpt is from the original PyroManiac blog back in September 2005. Phil explains why this distinction is vital, and how it is rightly applied.
We've been talking about the distinction between (on the one hand) truth that is so essential to the gospel—so vitally important—that you must affirm it or be condemned; and (on the other hand) lesser truths, where there may be more room for misunderstanding or disagreement. How does one tell which category any given doctrine fits into?
Some have suggested (and I quite agree) that Scripture may be deliberately vague on these issues for good but hidden reasons, so that some of the questions we have raised are answered in the Bible with stark black-and-white clarity; while most of the answers we're looking for are sketched out in indistinct lines and with varying shades of gray.
On the other hand, some who have commented have wondered aloud whether any distinction between essential and peripheral truths is really even necessary.
It seems to me that even a few moments of cursory thought would quickly drive us to the conclusion that we cannot simply erase every distinction between primary and secondary doctrines...Scripture commands us to contend earnestly against error when the faith once delivered to the saints is at stake; and yet, when the unity of true saints is at stake, we are commanded to receive people who are weak in the faith without indulging in doubtful disputations.
We're expected to make sound judgments about which is which. Remember, Jesus sternly rebuked the Pharisees for failing to distinguish between vital and secondary legal principles—even though no distinction between "gnats" and "camels" was ever spelled out explicitly in the Old Testament law. They were held responsible to apply rational, sensible judgment to the biblical data—and there was plainly enough data so they should have understood that justice, mercy, and the love of God are bigger spiritual principles than counting out little seeds for a tithe (Luke 11:42).
Notice what Jesus said: "These you ought to have done, without leaving the others undone" (Matthew 23:23). Recognizing a proper taxonomy of spiritual principles doesn't give us permission to abandon whatever principles are deemed secondary. I think that's a misunderstanding that causes some to shy away from speaking of "secondary" truths. But "secondary" doesn't mean "optional." It does, however, mean that all truths aren't of equal import. Not every point of truth is an occasion for all-out battle, especially between brethren who agree on the major points.
That's one of the huge practical realities of real-world ministry that sensible people who want to be faithful to the commands of Romans 14 simply must understand. We may not always agree on which issues are worth fighting for, but it's an evil mind that rejects the distinction completely and fights with equal vigor over every issue, gnats and camels alike.