God rarely works apart from human faith. Our trust and confidence in God's power, holiness, and goodness are God's standard means of working in the world. Similarly, salvation is by grace alone through faith alone. No faith, no salvation.
But where does that leave us? For on too many days, I want to throw in the towel. On too many days, my faith is feeble, fragile, and fragmented.
John Calvin rightly works through this issue in Book III, chapter 2, paragraph 7 of The Institutes of the Christian Religion: "But the human mind, when blinded and darkened, is very far from being able to rise to a proper knowledge of the divine will; nor can the heart, fluctuating with perpetual doubt rest secure in such knowledge [i.e. faith]. Hence in order that the word of God may gain full credit, the mind must be enlightened, and the heart confirmed, from some other quarter. We shall now have a full definition of faith if we that it is a firm and sure knowledge of the divine favor toward us, founded on the truth of a free promise in Christ, and revealed to our minds and sealed on our hearts, by the Holy Spirit."
Even faith is God's gift. I cannot manufacture faith. I cannot maintain faith. It is God's gift through the Holy Spirit. So even on the days when my faith is most fragile, I must remember that God will support and sustain me. In the darkest hours of my disbelief and in the valley of death, I need not fear evil for God is with me. Surely goodness and mercy will follow me all the days of my life and I will dwell in the house of the LORD forever.
"The grace of God does not find men fit for salvation, but makes them so." - Augustine