Lots happens in holy communion. Let’s just say this: The celebration of communion is a memorial of Jesus.
A memorial is not only a reminder to the participants (though undoubtedly it is that). More fundamentally, a memorial is directed at God. In the Supper, we memorialize the death of Jesus before His Father, calling on Him to keep the promises sealed with the blood of His Son.
How does this work? Again, much to be said. But at least this: We eat and drink now in the midst of enemies as a memorial that our Father has promised us a place in the marriage supper of the Lamb at the consummation of the ages. By memorializing the future in the present, we call on the Father to bring that promised supper to pass.
Or this: We eat and drink creation glorified, as a memorial of the promised birth of new creation through the birthpangs of the old.
Or this: We eat bread and drink wine as a sign that the Lord has delivered us from plunderers (Isaiah 62:8).
We could go on, but at least this.
How can this go wrong? We could refuse to eat and drink in the Father’s presence. Then the promises of a future feast, the promise of glorified creation, the promise of deliverance are not memorialized before the Father. And the kingdom does not come.
Or: We can gather, bring bread and wine before God, but refuse to eat and drink it (as happened in the medieval church, as happens in Protestant churches where the scrupulous refuse to participate). Then the promises of God are not memorialized. And the kingdom does not come.
We could go on, but at least this. And then this: We memorialize Jesus by eating bread and drinking wine. Unless we do that when we gather for worship, we’re doing something less than Jesus wants, and we are doing less for this world than God intends.