Faith comes by hearing, and hearing by the Word of God (Romans 10:17).
Some theologians like to talk about different kinds of faith. For example, the Puritan author William Gurnall in his classic book, "The Christian in Complete Armour", volume 3, talking about "The Shield of Faith" distinguishes "miraculous faith", from "justifying faith". He identifies the latter as the faith of which the phrase "the shield of faith" speaks. Gurnall points out that even Judas had "miraculous faith", or faith for miracles, inasmuch as he drove out demons and worked miracles along with the other disciples. Yet Judas was not saved in the end. Gurnall wants to say that Judas never truly believed and in support of this quotes John 6:64. But John 6:64 does not SAY that Judas did not ever believe, it merely states that Jesus knew those who did not believe, and [also] who would betray Him. One refers to a group, and the second to an individual.
John Wesley talks about "the faith of devils" as yet another kind of faith. Gurnall calls this "historical faith". The demons believe in God, and tremble (James 2:19). Certainly, this shows that there is a certain kind of believing in the FACTS coldly or even WITH TREMBLING (negative emotion) which does not count as saving faith. Real saving faith clings to Christ and humbly depends upon Him. I do not think we can say that this is all that real saving faith does – since James the apostle clearly teaches that faith without works is dead. What this really means is that your actions or lack of them will demonstrate the reality or otherwise of your faith in God. Abraham's saving faith began as a belief that God could give him many descendents (Genesis 15), but it grew and matured over time to make Abraham a man so trusting and loyal to God that he was willing to put his own promised son to death as a sacrifice, trusting that God could give him back.
Faith for miracles is a very useful kind of faith and it does create impact. It is fashionable amongst some Christians to despise this kind of faith, as if they are somehow above or beyond this sort of thing. But something is wrong when we take pride in the fact that we don't believe some of God's promises, such as the ones in Mark 16:17-20, Mark 11:23,24 – and other statements of Jesus. Sometimes it is just plain spiritual laziness which puts us in a place where we have no expectation that God is going to do anything miraculous to confirm the gospel. Worse yet, we can get in a place where we are not even interested in people hearing and believing the gospel. May God deliver us from such a state, if that is where any of us are at right now.
It IS possible to exercise faith for miracles and not be in possession of true saving faith. This is because you can believe in certain promises and operate even in certain "anointings" without truly trusting Christ, or submitting your life to Him. We don't want to live in this way, but neither should we desire to live in a place of emasculated Christianity, where we trust God for eternal salvation but treat the miraculous promises of God as things to be ignored, despised or rejected. Is there anything right about despising the methods of Jesus and the apostles? They used miracles to show forth the Kingdom of God. Shouldn't we? Don't the gospels strongly suggest that we should be carrying on the same kind of ministry as Jesus and the apostles – at least corporately as the Body of Christ?
There is no form of unbelief in Christ which is going to do you any good. While justifying faith and faith for miracles might be separable, God intends for them to go together in your life. After all, God put the same word "believe" in both Mark 16:15 and Mark 16:17. One talks about believing and being saved. The other talks about believing and seeing signs and wonders happen. Its such a shame that church people have such a history of dismembering the truths of Mark chapter 16. We would be a lot further down the track if people did not insist on doing this.
There are many many benefits to faith. Rather than analyzing which kinds of believing are necessary, and which are not, for our eternal salvation, I believe we would be better off growing in all aspects of faith – justifying faith, sanctifying faith, empowering faith – even "historical" faith for that too has its place. Quite simply, we ought to take our growth in God and the knowledge of God seriously – and developing as much faith as we can is a big part of that.