The word 'mystery' preserves the tension between the concrete and the divine. Something is definitely present, but what is present exceeds and overflows the limits of the concrete, even if it is present only by means of it. This is mysterious, in a way unique to Christian understanding.
Yep, I agree. Just because all we see are bread and juice (in our case), for instance, doesn't mean that is all that's there.
Later on p. 52 Galli says...
A minister says words and performs actions, but at a deeper level, it is Christ who is presiding. We share in bread and wine, but the reality is that we are taking Christ into us. It looks like this is all occurring in time and space, when in fact the boundaries of time and space are being shattered, when for a few moments "heaven and earth are full of [God's] glory."
When all is said and done, though it may look like we've done nothing more than re-enact a routine religious meal, in fact, as the concluding prayer notes, something terribly significant has occurred: "You have graciously accepted us as living members of your Son our Savior Jesus Christ, and you have fed us with spiritual food in the sacrament of his body and blood."