Start with the Reformation. You have printed Bibles around the place. You have corruption in the Church. So you start saying:
The Bible is truth!
This gives you a lever to overthrow the old idea, which was perhaps even often unstated: tradition is truth. A new truth-claim lets you unsettle the old world.
After that initial crowbar job, other things pile in with their claims. Hence:
That which is discovered by reason is truth.
That which is established by the Scientific Method is truth.
Establish all the facts and you establish the truth!
And because humans are complicated and clever:
All claimed ‘truth’ is just a way of bullying people and all claims to truth are simplistic and over-ambitious.
The truth we believe is a construct inside our head. The objective truth outside our head is the only truth but it is forever unknowable since we can only know what has made its way into our head, and that which has found its way inside our head is only a tiny unrepresentative subset of the ‘real’ truth outside.
Or what about
Truth is actually found in music or art or poetry, a chimeral thing that we occasionally encounter, but never grasp, and obviously beyond words.
Worse, I can’t think of a reliable way of judging between all these competing claims. How can you test the truth of truth? Though there are workarounds. For example, I prefer my Ryanair pilot to believe her flight instruments rather than her inner aesthetic sense. And if the air traffic controller said she was coming in too low, I would rather she believed him than accused him of abusing power for his own sexist reasons.
Where this gets us
I don’t know where this gets us, but I do think those who slickly think they have this whole Truth business nailed — the sort of people who say, ‘I’ll deal with your questions, just give me a moment’ — might be missing something.