After a baby is weaned she's taught to hold a bottle or cup. She must learn how to hold it, as important as learning how to feed herself. Once she's mastered holding the cup she must be taught that some things are good to drink and other things are harmful. It is the same when we are given the cup at the Lord's Supper.
At our Savior's table, we drink from the cup of salvation. We also learn about God's wrath at the same time. The cup we drink from gives us forgiveness, new life, and eternal salvation. This is possible because Jesus drained the cup of God's wrath. Jesus received the cup of God's wrath with the words, “Father, if you are willing, remove this cup from me. Nevertheless, not my will, but yours, be done." (Luke 22:42) Jesus drank the cup of God's wrath so we can drink from God's cup of salvation.
When we receive the cup of salvation we are assured, promised by God's own words, that under the wine Jesus' blood is present for our salvation. If we reject Christ we choose to drink from the cup of God's wrath. That's why the psalmist confesses, "it is God who executes judgment, putting down one and lifting up another. For in the hand of the Lord there is a cup with foaming wine, well mixed, and he pours out from it, and all the wicked of the earth shall drain it down to the dregs." (Psalm 75:7-8)
In the Garden of Gethsemane Jesus prayed that the cup be taken from him. He knew what it meant to accept the cup of God's wrath. It would harm him in ways we can't fathom. But, if he didn't drain the cup we would be doomed. Jesus received the cup of God's wrath for us - the cross, payment for our sin, God-forsaken, damned to hell - so that we can tip the cup to our lips and drink eternal life.
We drink from the cup of salvation at the Lord's Supper because we believe what Jesus says, that, "This cup that is poured out for you is the new testament in my blood." Also, when we drink, we proclaim to a world choking on God's wrath, that Jesus "took the cup, after supper, saying, “This cup is the new testament in my blood. Do this, as often as you drink it, in remembrance of me.” For as often as you eat this bread and drink the cup, you proclaim the Lord's death until he comes." (1 Corinthians 11:25-26)
When we proclaim Jesus' death we are, at the same time, preaching that this cup from which we drink is the cup of salvation for all who believe and receive it. That's why all Christians thirst daily for the only cup that truly refreshes us.
Jesus drank the cup of God's wrath for us on the cross so that each day of our life we may proclaim His death and resurrection, as we take the cup of salvation and call on the Name of the Lord.