But the blessing Christ promised, the blessing of great reward, is a reward of grace. The blessing is promised even though it is not earned. Augustine said it this way: Our rewards in heaven are a result of God's crowning His own gifts.
R. C. SproulThe cross was a glorious outworking of the grace of God, by which the Father commissioned the Son to make full satisfaction so that sinners might be saved with no sacrifice of God’s justice.
R. C. Sproul