“The way to fight sin in our lives is to battle our bent toward unbelief. We are prone to drift away from a hearty confidence in who Christ is, what he has done for us, and all the promises that are sure because of Christ. We must never let go of the blood and righteousness of Christ as the ground of our right standing with God and guarantee of all God’s promises.”
“There are church-attending people who believe they are saved because they once prayed to receive Jesus, not realizing that the genuineness of that experience is proved by endurance: “The one who endures to the end will be saved” (Matthew 24:13).
Battling Unbelief by John Piper (page 89-90)