Thou believest that there is one God; thou doest well: the devils also believe, and tremble. James 2:19.
Some people say you can't take the position that it would be hard to be saved when there are texts such as Acts 16:31 in the Scriptures, "Believe on the Lord Jesus Christ, and thou shalt be saved." Some people take the position that believing is all that's necessary. They say that if sometime during my life I have come to a decision to believe in the Lord Jesus Christ, then I am saved from then on--no problem, nothing to worry about, nothing to be concerned over, from that moment on.
Yet the Bible says that the devils do that much. They believe. They believe even a step further than some people believe. They believe that Jesus is the Son of God. They would even qualify apparently for John in his First Epistle, when he says that if you believe that Christ is come into the world, and that He is the Son of God, you will be saved (chap. 4:2). The devils in the days of Christ said, We know who You are. You are the Holy One of God. (See Mark 1:24.) That's even a step further than some people go today, right? So there must be something more than simply believing in a surface understanding of the word.
What about the people who confess with their mouth that Jesus is in their life, and someday will even say, Lord, Lord, we have cast out devils in thy name, and prophesied, and done many wonderful things (see Matt. 7:22). And they still aren't known by God. The person who will take Romans 10:9, 10, and say, All you have to do to be saved is believe in the resurrection and confess that Jesus is Lord, still has to consider other scriptures.
Actually, the truth is that the deeper meaning of he word translated "faith" or "belief" is "trust." The devils believe, but they don't trust. And that's where the difference lies. It is possible for me to believe that there is a God and believe many things about Him, just as the devils do, but not trust Him. How wide can we open the strait and narrow gate? How much room can we make? Jesus says, "Come unto me, all ye that labor and are heavy laden, and I will give you rest" (Matt. 11:28). It is the coming to Him, and continuing to come, that develops the trust in Him. That is more than just a mental assent.