The words sola gratia mean that human beings have no claim upon God. That is, God owes us nothing except just punishment for our many and very willful sins. Therefore, if he does save sinners, which he does in the case of some but not all, it is only because it pleases him to do it. Indeed, apart from this grace and the regenerating work of the Holy Spirit that flows from it, no one would be saved, since in our lost condition, human beings are not capable of winning, seeking out, or even cooperating with God’s grace. By insisting on “grace alone” the Reformers were denying that human methods, techniques, or strategies in themselves could ever bring anyone to faith. It is grace alone expressed through the supernatural work of the Holy Spirit that brings us to Christ, releasing us from our bondage to sin and raising us from death to spiritual life.
James Montgomery Boice, Whatever Happened to the Gospel of Grace? (Wheaton, Ill.: Crossway, 2001), pp. 65-149.