Marcan scholars point out a small, but significant action in today's gospel passage. Many people overlook it when they hear his version of Jesus' "words of institution." "Jesus took a cup, gave thanks and gave it to them and they all drank from it."
Given the context of a Passover meal, everyone sitting around the table with Jesus that night had his or her own cup in front of them. According to the Seder ritual, they took several ritual drinks from it. But at this point in the meal, Jesus departs from the official rubrics and tells them not to use their cups for the next drink. They're to drink from his personal cup, the cup over which he said the words, "This is my blood of the covenant, which will be shed for many."
Of course, when Jesus employs the term "blood of the covenant," he can only be referring to the scene depicted in today's Exodus reading. "Taking the book of the covenant, Moses read it aloud to the people, who answered, 'All that Yahweh has said, we will heed and do.' Then he took the blood and sprinkled it on the people saying, 'This is the blood of the covenant that Yahweh has made with you in accordance with these words of his.'"
Those who simply regard the historical Jesus as the founder of a new religion often forget the obvious: at the Last Supper Jesus is a Jew. As such, he uses Jewish symbolism to convey the meaning of what he's about to do. The cup which passes from disciple to disciple contains more than just his blood. Those who drink from it are agreeing to carry out the same covenant which Jesus has made with Yahweh. Just as the blood splashed on their ancestors at Sinai was an outward sign that they had made the covenant with Yahweh, so drinking from Jesus' cup becomes the outward sign that his followers are committing themselves to carry on Jesus' ministry. I presume if anyone around the table in the Jerusalem upper room that night refused to drink from Jesus' cup, he would have suggested that he or she might more profitably spend the evening eating and drinking somewhere else. He expected anyone who drank from his cup to imitate his value system.
The author of the Letter to the Hebrews treats a different aspect of Jesus' blood. For him it's not the outward sign of a covenant, it symbolizes something with which we're more familiar: it's the blood which guarantees the salvation which the covenant promises, a new covenant which Jesus makes part of our lives. Though this insight into Jesus' blood fits the both/and pattern of Semitic thought and theology, we shouldn't let it distract us from the point Mark is trying to put across.
Because most of us were baptized as infants, we frequently long to make an adult commitment to Jesus and his faith, something only those baptized later in life can do. The late Bishop Fulton Sheen once tried to turn the sacrament of confirmation into such a commitment. As bishop of Rochester, NY, he refused to confirm anyone not old enough to have graduated from high school. He reasoned if people were confirmed at a younger age, many would do so just because they were part of a "confirmation class," not because of a personal commitment to the faith of Jesus.
Sheen really didn't have to resort to such a drastic action. Following early Christian theology, we already have an adult commitment sacrament. Every time we take from the cup we're proclaiming to ourselves and everyone around us that we're giving ourselves over to Jesus. According to today's gospel, consuming Jesus' blood of the covenant isn't something we do for "extra credit." It's an essential Christian ritual. Refusing the eucharistic cup means either we don't know what the ritual means, or we're not about to commit ourselves to carry on Jesus' work.