A simple illustration will help us understand. Imagine being the root of a tree. The root does a tremendous amount of work in producing the tree's fruit. It draws moisture and nourishment in the form of minerals from the soil, processes them to some degree, then passes them on to the rest of the tree. However, the root could do none of this unless the minerals and water were freely given and available for it to do its work.
Similarly, faith is a gift, like the water and minerals, freely given by God to produce certain works. Like a tree root, we have a measure of control because we have a part to play—working to believe and use what God has given—in producing the fruit of the Spirit. Thus, faith is simultaneously a gift and a work. It is a gift because the Son of Man gives it to us (John 6:27) and a work because we must exercise it (John 6:29). As a food, faith is very nourishing for the mind, necessary for mental soundness and an abundant life. It will not be present in us unless we work to assimilate it by believing it.
John W. Ritenbaugh
Eating: How Good It Is! (Part Four)