Grace is God’s undeserved, free gift of mercy and forgiveness to sinners, by which He declares them righteous solely on account of the works and merit of Christ.
Denominations in Christianity understand “grace” in a variety of ways. See how the Scripture-based understanding Lutherans have of grace compares with others.
My salvation is as certain as Christ, because everything Christ did is mine! You see, it’s blasphemous to say, “Yes, I believe in Christ, but I don’t know if I’m really going to heaven.”