To some folk the very idea of there being primary and secondary truths is anathema and smacks of compromise! "Everything in the Bible is true, so how can you say some is primary and some is secondary?"
And there is something to be said for this view, for all Scripture is God-breathed.
The source of the problem
The problem is not with Scripture, the problem is with us. The problem is that there are - in the wisdom of God - some truths that every believer understands as central or fundamental, and there are other truths on which they find they disagree with their brothers and sisters. For example, all believers believe that Jesus is returning one day, but exactly how he is returning is a subject on which they differ.
So a believer who says "all the truths in Scripture are primary" is simply saying, "all the truths I believe are primary". I think we can all sniff pride here.....
We - even as believers - are fallen sinners. And there are many factors that play into our understandings of secondary doctrines, such as pride, what books we've read, what preachers we've listened to, what backgrounds we've come from - even our ability to think and reason.
We're not to blame Scripture - indeed there is only one set of truths there - but there is not one believer on earth who is the perfect interpreter of Scripture (remember we don't believe in popes?)
Ways to discern primary from secondary
This is no new debate, of course. The early church fathers wrestled with it and came up with creeds in an attempt to codify the central indisputable truths.
One way to discern a primary truth is how often it is repeated in Scripture. For example, that God is the Creator of the universe is stated so frequently that to deny it is to be a heretic. (You will notice that I did not add created the world in 6 24-hour days - why? Because on detail Christians - real ones - are genuinely divided).
Another way to discern primary truth is this: sometimes the Scriptures themselves say "this is of first importance". Take for example 1 Corinthians 15:3, "For what I received I passed onto you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day..." People who deny that Jesus died for our sins, deny that he was buried and died, deny that he was raised from the dead are heretics. It really is as simple as that.
A third way of discerning primary truth is to ask what the church has held down the ages. We are not on our own, we are privileged to be in a long line of believers stretching back 2000 - no more - years. What did they believe? Creeds are not irrelevant.
The so-what question
Once we have discerned primary from secondary the hard part begins. Because the only conclusion to such a process is to love and cherish all those who hold to these primary truths and treat them as brothers and sisters in Christ.
For some this is too great a cost, and so they resort to "all my truth is the truth" "there are no secondaries" which helps them to justify living in their ghettos and not bothering with their "wacky" brothers and sisters in Christ.