Among Communion-taking American Catholics, only a small fraction would count as true believers--the sort who reject contraception, living wills and orgasm-enjoying; those who would instruct a gay child to prepare for lifetime celibacy; people who believe that Jesus monitors their every thought--and that they were born depraved but that another person accepted punishment on their behalf (and in whose debt they today stand); people who believe that Jesus’ authority is expressed through the Pope--and that one should never voluntarily entertain a sexual fantasy.
Almost no one in the pews seriously embraces these positions; a sociological observation becomes apparent: The Magisterium does not accurately predict the real beliefs of Communion-takers. Of a random 100 at suburban Twin Cities Catholic churches, you'll find almost no one actually qualified to take the sacrament. So attending Mass entails briefly pretending to be an uncritical adherent 'out of respect' for the presence of the theoretical true believer sitting next to you.
IOW: I honor your feigned belief; you honor mine. (Within the Rumsfeldian foursquare: the Catholic's Unknown-Known.)
Commenters unironically employ the phrase putting oneself 'out of Communion'. The phrase implies a flock in lockstep with Catholic doctrine: This claim is obliterated by empirical research. Church-going Catholics are already 'out of communion'--but take Communion anyway--and no one seems to care, save for a tiny minority.
In taking Communion, one ceremonially asserts a purity claim about oneself, for significant others' visual benefit. The Catholic church-goer subconsciously negotiates a 'reasonable range of departure' [from doctrinal orthodoxy], which--since it isn't openly discussed--allows wide elasticity for suitably-discrete straying. (A Known-Known, I'm aware.)
Putting oneself 'out of Communion' implies the existence of some mass-level, consistent self-adjudication that precedes one's decision to get in line for Communion. Pshaw! (When asked to police oneself almost everyone hires a deaf-mute as his cop.)
If a Catholic priest rigorously sermonized on who should not be taking Communion, he could quickly reduce Communion taking by 90%. But priests are evaluated in part on church attendance: An obvious conflict of interest incentivizes priests to avoid frank messaging. As a practical matter priests never seriously review--from the pulpit--who shouldn't be taking Communion. It's just too expensive.
Thanks to Slavoj Žižek for this talk--in which he discusses the philosophical observation in Donald Rumsfeld's well-appreciated 2/12/02 quotation, among other things.