God is Love and Light. When He says a thing, it is. He is. He is infallible, inerrant, trustworthy, all powerful and leads us individually according to His own will.
That just pushes the question back one step: Which Christians are following the Holy Spirit's leading?
If you answer: "The ones who are following the Holy Spirit's leading", then you are basically saying:
"I don't know which Christians are accurately representing Apostolic teaching/doctrine; I just know that whichever Christians are following the Holy Spirit's guidance are the ones accurately representing Apostolic teaching/doctrine."
And I hope you see how unhelpful such an answer [i.e. non-answer] would be.
-A8