In the context of this verse, Paul is concerned that people were “thinking of themselves more highly than they ought to think.” His final remedy for this pride is to say that not only are spiritual gifts a work of God's free grace in our lives, but so also is the very faith with which we use those gifts.
This means that every possible ground of boasting is taken away. How can we boast if even the qualification for receiving gifts is also a gift?
This truth has a profound impact on how we pray. Jesus gives us the example in Luke 22:31-32. Before Peter denies him three times Jesus says to him, “Simon, Simon, behold, Satan has demanded to sift you like wheat; but I have prayed for you, that your faith may not fail; and you, when once you have turned again, strengthen your brothers.”
Jesus prays for Peter's faith to be sustained even through sin, because he knows that God is the one who sustains faith. So we should pray for ourselves and for others this way.
Thus the man with the epileptic boy cried out, “I believe; help my unbelief” (Mark 9:24). This is a good prayer. It acknowledges that without God we cannot believe as we ought to believe.