The first question was, "If all we have to do to be justified is believe, how much do we have to believe?" This is a non-trivial question, and it carries an enormous weight. From an abstract theological perspective, it's a question of some interest. From the perspective of a person who is desperate to know that his sins are forgiven, it's a whole different question. I took a stab at answering it, but I thought it would be worthwhile to answer that question here.
The most careful discussion of justification by faith in Scripture is found in Romans 4. There is an answer for us in the first five verses:
It's interesting to note that the whole of the Apostle's argument on justification in Romans is based on the Old Testament. That's because justification by faith alone (Romans 5:1) is Old Testament truth. The Old Testament saints were justified when they believed God, and so are we.1 What shall we say then that Abraham our father according to flesh has found?2 For if Abraham has been justified on the principle of works, he has whereof to boast: but not before God;3 for what does the scripture say? And Abraham believed God, and it was reckoned to him as righteousness.4 Now to him that works the reward is not reckoned as of grace, but of debt:5 but to him who does not work, but believes on him who justifies the ungodly, his faith is reckoned as righteousness. (Romans 4:1--5, JND)
The principle of Scripture is "two witnesses" (Matthew 18:16). And so the Apostle takes two Old Testament witnesses to the truth of justification by faith: Abraham and David. The true genius of taking these witnesses is only understood when we see them in relation to the Law. Abraham was before the Mosaic Law (Galatians 3:15--18), David was under the Law. Both were justified by faith. And so we see that the Law does not annul the principle on which God justifies: both those under Law and those not under Law are justified by faith.
In the spirit of "two witnesses", let's just establish the principle of justification by faith. Romans 4:5 and 5:1 are unequivocal statements that justification comes from faith alone: in fact, faith in contrast to works. Acts 13:38 & 39 is another witness:
But this is still Paul preaching, so let's find another witness. How about the Lord Jesus?38 Be it known unto you, therefore, brethren, that through this man remission of sins is preached to you,39 and from all things from which ye could not be justified in the law of Moses, in him every one that believes is justified. (JND)
Or the Apostle John?24 Verily, verily, I say unto you, that he that hears my word, and believes him that has sent me, has life eternal, and does not come into judgment, but is passed out of death into life. (John 5:24, JND)
The truth of Scripture, from start to finish, is the God justifies the one who believes. There is nothing to do, only Someone to believe.31 but these are written that ye may believe that Jesus is the Christ, the Son of God, and that believing ye might have life in his name. (John 20:31, JND)
But the question was, "How much do I need to believe?"
There is a simple answer, and some not-so-simple answers. Let's start with the simple: Romans 4:1--5 establishes justification by faith on the history of Abraham, specifically on Genesis 15. The verse it quotes (v. 3) is Genesis 15:6
If we examine the verse in it's context, we find that God promised Abraham a son. Abraham believed Him, and so God counted that faith as righteousness. As far as I can tell, that's all Abraham believed. He didn't have a very thorough theology, he sure didn't have a creed or a confession. Romans 4 comments on his belief:6 And he believed Jehovah; and he reckoned it to him as righteousness. (JND)
Abraham didn't believe in a complicated theology: he believed God when God told him he'd have a son. He believed God despite what he could see. He knew he was too old for children, he knew his wife was too old for children, but he believed God when it didn't make sense.17 (according as it is written, I have made thee father of many nations,) before the God whom he believed, who quickens the dead, and calls the things which be not as being;18 who against hope believed in hope to his becoming father of many nations, according to that which was spoken, So shall thy seed be:19 and not being weak in faith, he considered not his own body already become dead, being about a hundred years old, and the deadening of Sarah’s womb,20 and hesitated not at the promise of God through unbelief; but found strength in faith, giving glory to God;21 and being fully persuaded that what he has promised he is able also to do;22 wherefore also it was reckoned to him as righteousness. (Romans 4:17--22, JND)
As an aside, I've heard people try and put a Gospel message into Genesis 15. Like, they think God somehow showed Abraham that Christ would die for him. But that's not what Genesis 15 says, and it's not what Romans 4 says. God promised Abraham a son, and Abraham believed him. That's as far as Scripture goes.
So let's ask the question: how much do you have to believe? Abraham believed a lot less than the Gospel, but God justified him. Let's be clear: the issue is not what you believe, but Whom you believe. When you believe God, He counts it as righteousness.
Now, it's clear that an outright rejection of what God says isn't faith. If someone presents the truth of Scripture and you scoff at it, you're not believing. That seems obvious.
But the good news for the troubled soul is, "God doesn't want you to believe enough, He wants you to believe Him."
See (and this is the not-so-simple answer), this is the question of someone who's turning faith into a work. Scripture presents faith as the opposite of works: "to him who does not work, but believes" (Romans 4:5). Believing is the opposite of work.
If you're worried that you don't believe enough, then you're viewing belief as a work. You're taking the attitude of, "Just tell me what to do and I'll get right on it." But that's not faith, it's doing. There's nothing to do, only Someone to believe.
Here's the thing: faith in and of itself has no worth. It's not like faith is the one good thing we can present to God. God's not impressed by your faith: not by its size nor its intensity. The value of faith lies in its object. God is looking for faith in Himself, not faith as some abstract quantity. So there's no sense where you can believe enough, God is looking at what you have faith in. Abraham said, "If Jehovah said it, it must be true." That's all God is looking for.
The fact is that Scripture unequivocally teaches the God justifies the one who does not work, but believes. There is no condition added to it. There's no requirement for tears or contrition or penance or penitence. There's simply taking God at His word. Saving faith is, "God said it, so it must be true."