The Unbiased Unbeliever
Such people are hard to find. Most garden-variety atheists are not willing to play this game; most are extreme in their cherry picking. They will take whatever isolated passages are at their disposal and are advantageous with respect to the immediate discussion point. If you argue something is sinful, they will tell you that our holy book says to "judge not." If you argue we should be loving and tolerant, they will reply that our holy book instructs us to stone adulterers and we're just too pusillanimous to follow up.1
But occasionally you do come across someone who is, in this sense, of good faith and character. For example when I used to venture into Pharyngula, that cesspool of enslaved thought, unwavering dogma, irrationality, and confirmation bias, there was one commenter named owlmirror who was engaging and formidable in this regard. I miss dialoging with owlmirror.
So, for future reference (in future posts), I will have occasion to refer to the unbiased unbeliever, and will you refer back to this post.
The Biased Believer
Take the doctrine of the Trinity. There is a certain "degree of certainty" that we can demonstrate the Trinity from the text, and is not (in my opinion) 100%. Let's pick a number out of our derrieres, and assume that we can demonstrate the Trinity at, say, the 70% level of certainty. Meaning that if we have an intelligent unbiased unbeliever, she would tend to agree that the Christian holy book supports the doctrine of the Trinity at around that level of confidence. (We are not asking her to believe the doctrine, just evaluate how much the text supports it.)
For we as Christians to believe this doctrine (as opposed to just accepting it as plausible) we must either delude ourselves or, I would prefer to think, are biased through the undecipherable uttering and urging of the Holy Spirit. There is a voice in our heads, if you will, telling us to go ahead, believe!
So in this made up example, our belief in the Trinity is 70% derived from the text, and 30% from Holy Spirit bias.
The question I'd like to propose in the future is this: Is there a threshold on the "derived from text" part. Are there doctrines that are 2% text driven and 98% indemonstrable from the scriptures?
1 This is codified here. See Law 14: The Law of the Atheist Hermeneutic and Law 15: The Ruby Tuesday Law.