One answer the NT gives us is this: you don't really have to know anything to be saved. That is, you can have the faith of a little child, and God will welcome you (cf. Mt 18:1-6). You can have a simple faith, a milk-drinking faith (cf. 1 Cor 3), and be saved.
But there's another piece of the NT which frequently gets soft-soaked, and it's the answer which James gives: while a simple faith saves, it does not save only in the eternal sense. That is: it saves you to maturity:
the testing of your faith produces steadfastness. And let steadfastness have its full effect, that you may be perfect and complete, lacking in nothing.That is, your simple faith is also a living and breathing faith which grows you through trials to a "complete faith."
Many folks read this – rightly, btw – to mean "a right faith does works", and that's fine. That's a good application. But is it the only application? Is it the only one James intends here?
For example, when James says,
But be doers of the word, and not hearers only, deceiving yourselves. For if anyone is a hearer of the word and not a doer, he is like a man who looks intently at his natural face in a mirror. For he looks at himself and goes away and at once forgets what he was like. But the one who looks into the perfect law, the law of liberty, and perseveres, being no hearer who forgets but a doer who acts, he will be blessed in his doing.isn’t James saying that God's word is there so that we can take action upon it, and learn how to live in faith?
And glancing up this post a second, isn’t it also Paul's point in 1 Cor 3 that the Corinthians ought not to be forever babies in the faith, but that eventually they have to move on to the meat of the word? That is: their faith ought to make more of them, and be more to them than (as Paul implies) baby food.
So in that, there is a second answer to what you ought to know to have a saving faith: it ought to be true, and correct, insofar as you are mature and maturing in your faith.
What Scripture teaches us is what we must accept as the truth about our faith. And as we advance out of spiritual immaturity to spiritual maturity, the burden upon us to accept and demonstrate the truth in Scripture becomes a greater responsibility. This is why the warning to teachers is such a serious thing; that's why the anathema against a different gospel – and the criteria for knowing what that is – is an anathema and not just a rebuke.
And for good measure, think about this: that's why John called the Pharisees who came to see him a brood of vipers, and why Jesus called the same men whitewashed tombs -- because the Gospel had not changed, but these men, who ought to have known better, did not know it when they saw it.
You don't need a perfect confession to save you, but you do need a faith which is perfecting you, not leading you into more error.