Let us, then, understand the calling whereby they become elected,not those who are elected because they have believed, but who are elected that they may believe.
Augustine-A Treatise on the Predestination of Man
That being said, it doesn't negate the purpose of the Lord's Supper. Paul is very clear and, to my knowledge, this is the only place in scripture that specifically states the purpose; we proclaim the Lord's dead and return.
markomalley: I am confused, though, why you assume that John 6:64-65 would be problematic for a Catholic? Of course, God knew before the foundation of the world who would be saved and who would not be saved.
HD: According to Augustine, this is the definition of Pelagianism.
...Let us, then, understand the calling whereby they become elected,not those who are elected because they have believed, but who are elected that they may believe.
That is not what the Church taught. What the Holy Catholic Church (East and West) taught everywhere and always is
There is no "guesswork" when it comes to God's foreknowledge. But His foreknowledge does not take away our free will which was clearly given to us in the Garden of Eden.