Let us appeal to what the bible has to say about faith.
The first passage I recommend is Hebrews 11. I suggest reading the entire chapter, but vv. 1-3 are particularly important:
【11:1】 Now faith is is the assurance of things hoped for, the conviction of things not seen.
【11:2】 For by it the men of old received divine approval.
【11:3】 By faith we understand that the world was created by the word of God, so that what is seen was made out of things which do not appear.
The chapter goes on to describe the faith of the Patriarchs like Noah and Abraham. In a nutshell, faith is a gift of trust in God’s existence, His power and His unfailing love. Faith is both a gift from God for us to believe God whom we cannot see and our response in trust to follow Him.
In John 14, Jesus repeatedly appeal to us to believe in Him, describing faith as an intimate trusted relationship with Him:
【14:1】 “Let not your hearts be troubled; believe in God, believe also in me.
【14:2】 In my Father’s house are many rooms; if it were not so, would I have told you that I go to prepare a place for you?
【14:3】 And when I go and prepare a place for you, I will come again and will take you to myself, that where I am you may be also.
So we can see faith primarily is a call to trust God and Jesus, to enter into a deep trusted relationship with Him.
Faith, elsewhere, is also understood as a power to act supernaturally. As many times when Jesus was performing a miracle, He asked the person if he believes He can heal him, or pronounced that “Your faith has saved you.” This is also evident in His well known expression in Matthew 17:20:
if you have faith as a grain of mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible to you.”