Faith is the noun form, and believe is the verb form of the same Greek word. This makes believing the action side of faith, but you cannot act in faith until you have faith. Faith is the ability to believe, but that ability must be acted upon for it to work. Believing is the action of faith. Just having faith is not enough. James tells us that faith without corresponding actions is lifeless and dead.
You see that faith was working with his works, and as a result of the works, faith was perfected; James 2:22 New American Standard Bible
You see that faith was active along with his works, and faith was completed by his works; James 2:22 English Standard Version
You can’t have strength without a muscle, and you can’t drive without a car. You can have a muscle and not lift a weight, and you can have a car and not drive it. Having them gives you the choice of using them. If you did not have them, you would have no choice. Without having faith in the heart, we are not able to choose to believe. Believing is only an option once you have faith in the heart. And faith only comes by hearing from God.
Many times we try to believe before we have faith. We may even fool ourselves into thinking we have faith and that we are believing. We call this condition mental assent. It means that we are only mentally ‘assenting’, or agreeing, to the promise of God and not fully believing it in the heart.
Faith is of the heart and faith comes from God. The Bible says that faith comes from God (Romans 10:17). It comes when the word on a certain subject comes alive in our heart. Now, if it comes, then there must be a time before it comes. One moment one does not have faith and the next moment they do have faith. You have faith or you don’t; it’s that simple. When one has received faith, James 2:22 says that, they must act on that faith by believing, if not it will become nonproductive. Believing requires faith, and faith requires the action of believing.