This is a brilliant question, and the New Testament gives us a two-fold answer. And that is itself a brilliant answer because that means the truth is not a checkbox or a light switch: it's not either on or off but a nuanced distinction which sets it apart vividly from error.
One answer the NT gives us is this: you don't really have to know anything to be saved. That is, you can the faith of a little child, and God will welcome you (cf. Mt 18:1-6, for the proof-texters and OGs everywhere). You can have a simple faith, a milk-drinking faith (cf. 1Cor 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.
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.
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 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.
Here's what I mean by that – by way of example. Let's think about math for a second. My son loves math (thank God – please Jesus make him a man who has a heart for God and people who is an accountant), and we are working flash cards to learn how to multiply. He can add, he can subtract, and now he wants to learn how to do "times". Which is great, if you ask me: he ought to learn how to do "times".
The other day, he asked me, "Daddy, do you do math at work?" And the answer, of course, is yes – I do a lot of math at work, a lot of it requiring advanced algebra. So I told him, "yes, son: I do a LOT of math at work."
"Can you show it to me?" he asked.
Well, sure I can show it to him – so I open up my laptop, open up some spreadsheets, and I show him the greek-like formulas we have either borrowed or invented to discover things like how many dollars we are earning per hour, given the rate of production vs. the standard work for a given work center. And then there's the statistical stuff I have to do verify and compare forecasts. And then there's the financial comparisons vs. plan and vs. last year. And so on. (hey: wake up. The boring part's over)
So he says to me, "but where are the numbers?" See: in his understanding of math, you need two numbers to make an equation, and those two numbers yield a fixed answer – which, factually, is the right view of arithmetic, and ultimately the right view of how a formula yields an answer you can use.
So I tell him, "Son, we fill in the numbers when they come by. This kind of math shows us how to think about certain problems, and when a problem comes up, we change out the letters for numbers to get an answer."
"WHAT?!" he yells, sort of laughing. "Daddy, you can't add up two letters!? You can't add 'A' plus 'B' and get 'C' – they're LETTERS!"
Well, really: he's right. Even in algebra II, the formula gets solved down to its simplest state, or most useful state, and you don't really get numbers at the end – you get formulas. But understanding that requires a leap from linear, arithmetic thinking to something more conceptual – something which is taking in the big picture of how adding 2 + 2, or making 3 "times" 4, works.
So my son can have a completely –correct- view of arithmetic, and be –unable- to grasp algebra yet. That doesn't make his view of math "false": it makes it incomplete. He's not a heretic to the math community: he's a student. His view is correct insofar as it is advanced, but it doesn’t account for all of math.
Now, if in 10 years my son and I sit down and he says to me, "Dad, open up your laptop for me – I want to see what you're doing at work," I'll be glad to oblige. My fatherly optimism will be that he's just completed Trig and he's about to show me how to simplify some of my 3-legged-dog formulas into something a little more sleek and functional.
But if we open up the laptop and when he looks at the spreadsheets he says to me, "You know what, Sir Dude? [he uses 'sir' out of respect because he was raised right] I still don't buy the algebra thing. I know what you call it – I just don't buy it. It doesn’t work. 2 + 2 = 4; A + B doesn't equal anything. All this stuff you say you've been doing for the last 10 years is just guff. And there's no way for you to prove to me that it does work."
At that point, we have crossed over from incomplete knowledge to something else – a knowledge which refuses to grow, refuses to receive more. It's willful ignorance.
In Biblical terms, it's what Paul called the "shipwreck of faith" – that is, when some rejects "love that issues from a pure heart and a good conscience and a sincere faith." "Love" is certainly the product, but one of the components of that love is a "good conscience". In my example, my son can’t be said to have a good conscience – because he establishes what cannot be true apart from the facts which are plainly in evidence. In our faith life, we cannot be said to have a good conscience if we are unwilling to receive the facts of faith.
That's a big deal, for example, for Catholics – because the right-minded Catholic view is that Protestants who willfully refuse the teaching of the Church are unrepentant sinners. And if they are right about what kind of final authority the Church as an institution holds, they're right: you can't have a good conscience with you reject the truth.
But for Protestants – not merely evangelicals, but confessional Protestants – 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.