So in high school I was dating a girl whose father was a Methodist minister. One day I went to her family’s church to play trumpet for the choir. The service was good, they had a communion service (no, I didn’t partake) and the music went well. I was sitting in the sacristy after cleaning my trumpet when her mother came in bringing the communion that was left over from the service. She tilted the silver platter and dumped it all into the trash.
I had a moment of freaking out - like seeing someone bumping a glass full of water and reaching out to catch the glass before it spills. Then I realized that she had done it on purpose and was embarrassed. She said something along the lines of, “Oh you Catholics,” with a kind and broad smile on her face, “it isn’t Jesus anymore.”
It is intellectually easier for a very good Methodist to open up their communion service to anybody who professes the Name of Jesus, even those who are not yet baptized. This body of Christians does not believe in transubstantiation but that in some way not fully explainable (it is a mystery - understandable) that “the elements are essential tangible means through which God works.”
But let’s suppose that you see something different in Communion. You see and/or understand a Divine Person clearly present. It would be similar to the person who sees life in the womb as a cluster of impersonal cells, but you see a vital, viable and inviolable human being - a boy or a girl. Who would you entrust to carrying your baby to term for you: someone who promises to have respect for your clump of cells, or someone who has love for your baby girl?
If they have respect for your clump of cells, that may be grounds for a very good friendship. If they would love your baby girl the way you do, that is a grounding for marriage. And who is invited into THE MOST INTIMATE ACT of the marriage covenant? Those who are part of the covenant. Who is invited to Catholic communion? Everybody. All people of all times and all places. But if you have no desire or belief to be part of this mystical marriage in a permanent way, we can share all kinds of things together, just not this Person Who we believe is present to us Body and Blood, Soul and Divinity.
That is why paragraph 80 (yes! we have escaped paragraph 79) of the GIRM says that it is desirable that at the Paschal Banquet, in keeping with Christ’s instructions, “His Body and Blood should be received as spiritual food by the faithful who are properly disposed.” Properly disposed to receive means about the same thing as someone who is properly disposed to receive the sacrament of marriage. For marriage to be valid, a person must, know what they are getting themselves into, understand it, accept it, and fully give themselves to it and to a partner. Eucharist is no different. We are to know exactly what it is that we are doing, understand it, accept it, and give ourselves not only fully to Him, but to this Body of Christ, the Church, to which we are to be made one to be properly disposed. If not, communion (common union) does not really exist and what we act out is not in accord with the reality.