The Westminster Confession also teaches, rightly, that one of the purposes of the sacraments is to “solemnly engage” us “to the service of God in Christ, according to His Word.”
When we are “engaged” in this way it means that we are bound to Him by means of an oath. When we are baptized, we either take an oath, or an oath is taken on our behalf—that we will live for the service of God in Christ, according to His Word. Baptism is the oath that inaugurates the relationship—much like the oaths that a bride and groom take on their wedding day. That is the fundamental oath, that is the oath that is foundational to all others.
But our confession is talking this way, about our “solemn engagement” in its section on the sacraments, and so this means that it applies to the Lord’s Supper as well. Every time we come to this Table, we are coming to be renewed, and on that basis to renew our commitments, to renew our solemn engagements, to make our vows, to honor our promises.
Now this practice of “solemn engagement,” if we are not careful, can easily drift away from grace, and we will discover ourselves neck deep in moralistic scruples and vows, and no clear way out. But when we do this, it is not because we are keeping our vows, it is because we are not. What is the work that God requires of us? It is for us to believe.
We keep our promises by trusting in the only man who ever kept all His promises—the Lord Jesus. We remember our vows when we pray in Jesus’ name, the one who fulfilled all our vows for us. We obey God when we turn to the one who obeyed God perfectly on our behalf. We offer up recompense for our sins when we plead the blood of Jesus.
Look at the cup here. That is not your blood. Look at the broken body. That is not your body. You are not the promise keeper.
But there is something else, and this is the glorious part. That blood, which is not yours, in just a few moments, will be given to you. And when it is given to you, it will be yours. This broken body, which is not yours, is shortly going to be given to you. And when it is given to you, it will be yours.