Years ago experienced this in a church where one would expect that the Bible would be considered as the ultimate authority. In an adult Bible study we got to talking about thing that could be considered black or white. When something was brought up that the Bible is clear and loud on that we are to have nothing to do with. When someone brought up a series of books about this subject should be allowed in school because they are a very good form of literature and extremely well written. This person dismissed what the Bible said because it was mostly in the Old Testament. I was floored what happened to the Bible being God's Word and standard of living.
The Greek words for the word truth that is in 3 John: alétheia,- truth, but not merely truth as spoken; truth of idea, reality, sincerity, truth in the moral sphere, divine truth revealed to man, straightforwardness;
peripatéō - I walk, hence Hebraistically (in an ethical sense): I conduct my life, live.
What John is saying here is that we need to conduct out lives in a way that lines up to the divine truth that God has revealed to us through the Bible. We can't pick and choose what parts we are going to obey because they don't fit what we want to do, we have to walk in the whole truth.