The New City Catechism # 47: Does the Lord’s Supper add anything to Christ’s atoning work?

Answer:
No, Christ died once for all. The Lord’s Supper is a covenant meal celebrating Christ’s atoning work; as it is also a means of strengthening our faith as we look to him, and a foretaste of the future feast. But those who take part with unrepentant hearts eat and drink judgment on themselves.

Scripture:
1 Peter 3:18
For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God.