A spectrum of belief exists between our own thinking on the left and what God says on the right. On the far right is believing what God says---all of it. On the far left is not believing what God says---none of it---but believing what you think. In between is a mixture of those two. Closer to the left is some kind of mysticism that believes there is a God and the Bible is some kind of allegorical or mystical guidebook with a whole lot of decent teaching about which you can have your own opinion. Closer to the right is a belief in all the things that are important or easy in Scripture mixed with doubt about those things that you deem less important or more difficult.
Does the Bible Teach It?
We believe or we at least should believe what we believe, first, because the Bible teaches it. The Bible is the basis of or authority for what we believe or should believe. If it's not taught in the Bible, we don't have have basis of or authority for believing whatever it is. Romans 10:17 comes into play here, that faith comes by hearing and hearing by the Word of God. Jesus said His Word was truth (John 17:17). Our faith and practice come from or at least should come from the Bible. If the Bible teaches it, we believe it; if it doesn't, we don't.
In one sense, what the Bible teaches is all that we need for faith and practice, so it is the only reason why to believe something. Scripture is completely sufficient for all faith and practice. So why not stop here? We don't stop here because the Bible, which is sufficient, gives us some other reasons to believe something. Why would it do that?
I want to illustrate. 1 Corinthians 11:3 says the man is the head of the woman. Most people would think that means he is in authority over the woman. However, certain "theologians" went looking for another meaning and recently have "discovered" that "head" really means "source." So the Bible isn't teaching that a man is a woman's authority, but her source---entirely different meaning. So here is the Bible as an authority, now saying that the man is the woman's source, not her authority. Both are not true. The Bible means only one thing; it can't be both. Both sides, however, are saying that the Bible is their basis of and authority for belief in this. This is when something else besides the Bible comes in.
At this point, someone may say, it is the Bible, rightly divided (2 Tim 2:15). Nice. And true. But both sides say they are rightly dividing it. They can both argue about why each is right. At least one is going to be wrong. We can judge whether the biblical arguments are sound---usage, meaning of words, comparing scripture with scripture, internal and greater context, etc. I understand that nothing might settle this, because people all over differ on a number of different biblical doctrines or issues. However, there are other aspects to consider.
Have Christians Believed It?
We also believe or we at least should believe what we believe, second, because other believers have believed through history what we see that the Bible is teaching. I do believe this is secondary, because history isn't inspired. There are two biblical bases for this. First, no total apostasy. We have biblical authority to say that God would preserve right belief in the age in which we live. Jesus said the gates of hell would not prevail against His church (Matthew 16:18). Paul said that only some would depart from the faith, not all (1 Timothy 4:1). The total apostasy will start only upon the appearance of the antichrist (2 Thessalonians 2), and he hasn't made his appearance yet. So we would expect that true doctrine would continue in this age in which the Holy Spirit continues in and working in the hearts of believers. Second, no private interpretation. The Bible means what it means. It doesn't mean what it means "to you." It means what it means whether you ever existed. You can count on other people believing it if it is true.
There is something scientific to this in the true sense. The Holy Spirit indwells believers. He is the Spirit of truth. He inspired Scripture. He knows what the Bible means. Someone who has the Holy Spirit is going to know what it says.
We don't operate according to this point about the Holy Spirit's illumination and the history of doctrine by saying that the right doctrine will be believed by a majority of people who profess to be Christians. Instead, we assume that Christians would believe it. True doctrine will also be historic. If we can't find other Christians professing a teaching of Scripture through history, we should have a tremendous exegetical basis for that teaching. And if we don't, we should consider that teaching to be wrong.
Is there Agreement from the Church?
A biblical grid for teaching is the church. The church is the pillar and ground of the truth (1 Tim 3:15), not a single individual. The priesthood of the believer doesn't mean that you have liberty to hold a teaching in conflict with your entire church. You should listen to your whole church in your pursuit of understanding the meaning of the Bible. This is not to say that truth is by majority vote. It is to say that an individual member should take very seriously the agreement of a whole church.
You can know the truth, despite your own frailty and depravity, just like you can know that you have eternal life (1 John 5:13). History and your church provide safeguards against new teaching. If what you believe is unique, you should question it and especially hold it up to the scrutiny of historic belief and the position of your church. Some might complain of some type of papistic elevation of church authority in subjecting doctrine to history and the church, but it really is just following scriptural guidelines that protect against apostasy.
Common Unbiblical or Extra-Scriptural Criteria
The relative popularity of a particular biblical teaching in the world does not guide whether you believe it or not. If people dislike it, that doesn't get to enter in as a basis for not believing. Feelings do not constitute a legitimate reason to believe or not to believe a doctrine or practice.
Going back to an earlier illustration, women may not like male headship. Female antipathy toward male authority does not alter genuine faith. If the Bible teaches a doctrine or practice, one's feelings are irrelevant to belief in the doctrine or practice.
Circumstances are another groundless standard for faith. Abraham faced all sorts of circumstantial barriers to faith---distance, age, barrenness, weakness, unpopularity. He instead kept believing God's Word.
We don't go to God's Word and say, "Well, women don't like to submit, so let's look for a different meaning for 'head' in 1 Corinthians 11:3." We don't go to God's Word and say, "Well, the earth looks really old, so we've got to rethink Genesis 1-3." We don't go to God's Word and say, "Homosexuals feel like they're born that way, so we've got to go back to the drawing board on Romans 1 and 1 Corinthians 5-6." We don't go to God's Word and say, "Lot's of people, who start out saying they are saved, end up living like the devil, so they must have lost their salvation." We don't go to God's Word and say, "We see variations in the hand copies (manuscripts) of Scripture, so God must not have made sure every one of His Words remained accessible to His people."
An Application and a Bad Example
Let's take the last one in the previous paragraph. We examine the Bible and we look at history and we get perfect preservation---original language, verbal plenary preservation of Scripture. God promised to keep every Word. And that's the position we see in history. So we believe it. We've got the necessary basis for faith and so we believe. That's how doctrine and practice work, at least how they should work. A few contradictory manuscripts, textual variants, or errors in copies don't undo the biblical and historical doctrine. The view of the manuscripts should conform to the doctrine, not vice versa.
In a recent and ongoing discussion or debate on the doctrine of perfect preservation of Scripture, someone who believes the doctrine laid out a relatively short biblical and historical presentation. His opponent attacked this as argumentum verbosium fallacy and proceeded instead to link to his own boatload of verbosity on the subject, leaving one with the impression of the acceptability of only linked verbosity---argumentum alinkum verbosium fallacy. He also suggested that everyone read a standard on the subject---and again his opponent claimed argumentum verbosium fallacy. You can't refer to a book, because that would be fallacious.
The perfect preservationist referred to Francis Turretin, who among the Protestants' is perhaps the greatest work of theology ever written. His writing represented the Protestant theological thought of his era. The idea of quoting or referring to Turretin is to report historical theology. It is not some desperate jump into obscurity. His opponent's answer: Turretin is just a man. That's true. Turretin is just a man. Historical theology is the writing of men. That is a convenient position for someone who has zero historical basis for his position. And he doesn't care, at least where he has no support, because, on the other hand, he doesn't mind dropping the translators' preface to the KJV even if it has no relations to a textual argument, only a translation one. He cares about that.
If someone has no history, then one would think that he would supply some great exegesis to overturn the historical position. That's not going to happen either. His primary exegetical argument is the depravity of man. Men are sinners, so they must have corrupted the text. I'm sure there's a logical fallacy in that argument. And his sharpest critique of the perfect preservation passages is that they could each have several different meanings, a few of which again conveniently succumb to the speculation that textual variants have ruined the possibility of a perfect text of Scripture. The circumstances drive the meaning, not the exegesis or history.
We believe what we believe because Scripture says it. We consider historic belief and practice as an application of the Bible. We don't allow feelings or circumstances or popularity to change that.