“While they were eating, Jesus took bread, and when he had given thanks, he broke it and gave it to his disciples, saying, “Take and eat; this is my body.” Matt 26:26
“So then, whoever eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of sinning against the body and blood of the Lord. Everyone ought to examine themselves before they eat of the bread and drink from the cup. For those who eat and drink without discerning the body of Christ eat and drink judgment on themselves.” 1 Cor 11:27-29
“Is” just means is. With the Eucharist we’re provided, for one thing, a means by which we can honestly face the question regularly as to whether or not we’re partaking of God worthily, whether or not the relationship is sound, whether communion with Him is valid or a mockery due to how we live. If we’re living by the flesh in some grave and gross manner that opposes love of God and neighbor, partaking would be a joke. He’s really there. That’s of great benefit for us to know.