And no, this doesn't mean your Sabbath School class needs to have a long, earnest discussion about how we know when we know what we know. Philosophers have spent the last hundred years hashing out the details of epistemology, remember -- and thus far, they haven't done much better than that old, old story about the blind men and the elephant.
Then too, I'd suggest you avoid the question of just how much "truth" someone needs to believe in order to be part of your particular group. In theory, this could be helpful -- but in practice, I've found that nothing makes people more set in their ways than a discussion of just how open-minded we should be.
Instead, you might want to point out that ἀληθείᾳ ("truth") can be more of an adjective than a noun -- in fact, it often used to describe someone who is honest, reliable, and trustworthy. When Paul said ἀληθείᾳ is a fruit of the Spirit, in other words, he didn't just mean Truth; he also meant truthfulness.
- If a mechanic says your car will be fixed by Thursday, in other words -- and it actually does get fixed by Thursday -- then that's ἀληθείᾳ.
- And if you have a witness on the stand who honestly, accurately, describes what she saw, then that's ἀληθείᾳ.
- But if you have a church where people "talk the talk" but they don't "walk the walk," then the people in that particular church may have plenty of knowledge . . .
And you can trust me on that.