Heb 11:6 puts it like this: "Without faith it is impossible to please him, for he who comes to God must believe that he is, and that he is a rewarder of those who diligently seek him."
Remember the woman with the issue of blood (Matt 9:20-22)? She'd been like that for 12 years, but she just knew if she could only touch the hem of His garment, she would be healed. She believed it when she went to meet Him, she believed it as she touched Him, and she continued to believe it after she touched Him. Jesus said to her "Your faith has made you well." Strictly speaking, you might want to argue, it wasn't her faith that had healed her, it was Jesus who had healed her. But it was her faith that enabled her to receive her healing.
The Bible is full of promises. It is faith that turns God's promises into personal experience. Faith is simply believing that God will do what He has promised to do. Faith is believing God's promise, despite the circumstances.
It's as though God looks for faith. It can be the faith of the person concerned; the faith of the people who are concerned about that person, like the people who brought the paralytic to Jesus (Matt 9:2); or the faith of others who pray, like the elders who pray for the sick person in Jas 5:14, 15. Where there is faith, miracles can happen (Acts 14:8 - 10; Mark 9:23).
Matt 21:22 says "All things, whatever you ask in prayer, believing, you will receive." Notice that "believing." Faith must be based on God's word. It's not faith in our faith, or faith in ourselves. Faith comes through God's word. "Faith comes by hearing, and hearing by the word of God" (Rom 10:17).
Notice how faith works. "Whatever things you ask when you pray, believe that you receive them, and you will have them" (Mark 11:24). According to that verse, when do we believe? When we pray. We believe that we receive the things we ask for, even though we don't see them yet. If we believe we receive them, that verse says, we will have them. In other words, we don't receive in order to believe, we believe in order to receive.
It is here that people miss it. They think about the need, say "Oh, I don't have faith for that," and don't pray. Don't do that. You may not have faith to begin with. Remember Rom 10:17. Faith comes by hearing, and hearing by the word of God. Faith comes. Faith isn't something you work up yourselves. Faith comes from above. Pray about the situation and continue to read God's word until faith springs up in your heart. Then you can ask and know you have the answer.
"God is not a man, that he should lie," the Bible says, "nor a son of man, that he should repent. Has he said, and will he not do it? Or has he spoken, and will he not make it good?" (Num 23:19). God is faithful to fulfil His word. Can you believe it?