If you really think about it, the final guarantor of anyone's beliefs is ultimately their own judgement.

Even if God spoke to you yesterday from a burning bush, you'd have to exercise some judgement on whether what you witnessed was real, and whether the voice you heard was, in fact, God, and not a hallucination, someone playing a trick on you, or something else.

Likewise, the choice to trust the red letters or the black letters is a judgement call.

The decision to consult the Bible at all is a judgement call.

After all, there are Muslims and Jews and Buddhists who all presumably should have equal access to the Holy Spirit, and yet choose to trust different books, friends, and teachers.

And there are people who have tried really hard to be good Christians, but who eventually become atheists or agnostics, because their best judgment just doesn't allow them to trust the Bible anymore.

At the end of the day, everyone truly is kind of alone inside their heads, and has to make their own best guesses as to how to interpret what they feel and witness.