How do we know the Lord's Supper is an ordinance of grace?

The Lord's Supper is indeed an ordinance of grace, distinct from the Old Testament Passover. While Passover involved a continual remembrance of the need for sacrifice, the Lord's Supper celebrates the sufficiency of Christ's once-for-all sacrifice. Hebrews 10:12 teaches that Christ's singular offering perfected those He saves, rendering any further sacrifices unnecessary. This supper is not a means of obtaining grace but rather a celebration of the grace already bestowed through Christ's finished work on the cross.