“The Reformers maintained that the sinner is saved by the grace of God, His unmerited favor, alone. This doctrine means that nothing the sinner does commends him to the grace of God, and that the sinner does not cooperate with God in order to merit his salvation. Salvation, from beginning to end, is the sovereign gift of God to the unworthy and undeserving. As Paul wrote to the Corinthian Christians, who were inclined to boasting: ‘Who sees anything different in you? What do you have that you did not receive? If then you received it, why do you boast as if you did not receive it?’ (1 Cor. 4:7). No one can ever stand before God and say, ‘Look at me and at what I have done!’ God is no one’s debtor, not least in matters of salvation (Rom. 11:35).” (Guy Waters)
For more on the subject, visit the Systematic Theology page.