The traditional Jewish celebration involved four cups—each representing something different, each with a different name. At various points during the ceremony a drink would taken from every cup. Every cup that is, except the third. This cup represented the coming Messiah for whom the Jews were in waiting. During Jewish Passover, the cup would remain untouched.
But when Jesus celebrated Passover with His disciples, this is the cup he picked up when He said “This I my blood shed for you.” This is the cup he told his disciples to drink from. Similarly, the bread he broke was a piece that was ceremonially set aside to represent the Messiah the Jews could not yet see. Jesus picked up both of these symbols, and said, “This is me. My body. My blood.” With those words, Jesus was declaring himself to be the Messiah they’d been anticipating every Passover up to that point.
Earlier in the post, this person said she attended the Christian Seder because of her desire to learn about Jewish culture. That’s great! Unfortunately, these Seders—whether they’re hosted by churches or “Messianic synagogues”—are offensive not only because they co-opt traditional Jewish rituals to deliver a Christian message, but because they distort the rituals to make it easier to deliver that message.
Had this person attended an actual Seder, she would have drunk four cups of wine; there’s no third cup left alone for the Messiah. (There is a fifth cup set aside for Elijah, but that’s another matter.) She would have also learned that the piece of matzah she mentioned, the afikoman, is merely set aside for eating at the end—and for children to steal in exchange for a reward, of course, in Ashkenazi communities.
There’s nothing wrong with wanting to experience a Seder, and many Jewish families and synagogues open their doors to others during Passover. In fact, half the people at the Seders I attended this year weren’t Jewish. There’s definitely something wrong, though, with distorting another group’s traditions in service of your own beliefs. I hope this person has the opportunity to attend an authentic Seder next year and experience the actual traditions.