Mark 14:22-24, "While they were eating, Jesus took bread, and after giving thanks he broke it, gave it to his disciples, and said, “Take, eat, this is my body.” And after taking the cup and giving thanks, he gave it to them, saying, “Drink from it, all of you, for this is my blood, the blood of the covenant, that is poured out for many for the forgiveness of sins."
It is incomprehensible to think that Jesus would be asking His disciples to drink grape juice to symbolically represent His blood. Wine has been part of the seder meal for thousands of years, as has been unleavened bread (matzoh). Wine, not grape juice.
Paul wrote to the church in Corinth, "Is not the cup of blessing that we bless a sharing in the blood of Christ? Is not the bread that we break a sharing in the body of Christ?" and "For this reason, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of the body and blood of the Lord." 1 Corinthians 10:16 and 11:27