The Westminster Confession of Faith refers to this sacrament, as with baptism, as "a holy sign and seal of the covenant of grace" (WCF, XXVII.1). One way to understand the idea of a covenant seal is to realize that the Passover was not merely a religious feast but was also a covenant meal at which God identified with his people, accepted them as his own, and spread before them his provision. We see this happening at numerous times in the Old Testament, but most notably at the ratification of the Mosaic Covenant. The Lord called Moses and Aaron and the seventy elders up onto the mountain, where “they beheld God, and ate and drank” (Ex. 4:11). That the Passover meal served as a covenant renewal is confirmed throughout the Old Testament, through Joshua’s celebration of Passover at Gilgal (Josh. 5:10) all the way to Josiah’s Passover renewal (2 Kings 23:21-23).

Therefore, when the minister today holds forth the bread and the cup of the Lord’s Supper, it is really Christ who sets the meal before his covenant people. Just as the blood marked the houses of those passed over by the angel of death, Jesus marked his disciples as his own, when he held forth the cup and said, “This cup is the new covenant in my blood” (1 Cor. 11:5). By means of his covenant meal, Jesus seals to his disciples the benefits of his atoning death, bringing us into the bonds of his new covenant of salvation.. Those who eat and drink of the Lord’s Supper, trusting in Christ, receive assurance of his blessing through this seal of his atoning work.

It is the government seal that makes a document official. Thus, the Lord’s Supper seals God’s people by giving them a reliable attestation of their participation in Christ. Christ thus identifies his own, stretching forth his hand to give them the bread and the cup of his covenant meal. John Murray says, “When we partake of the cup in faith, it is the Lord’s own certification to us that all that the new covenant in his blood involves is ours. It is the seal of his grace and faithfulness.” Berkhof points out that this sealing assures us that we are the recipients of Christ’s atoning work. The Lord’s Supper “seals to the participant the great love of Christ. . . it assures the believing participant. . . that he was personally the object of that incomparable love.” Furthermore, it confirms to the believer that all the promises of the covenant and all the blessings of salvation “are his in actual possession.” Lastly, it is a reciprocal seal whereby believers through participation "profess their faith in Christ as Savior and their allegiance to Him as their King, and they solemnly pledge a life of obedience to His divine commandments."