Guardian Angel or Angel(s)? - Calvin's Institutes 6

John Calvin:

"...angels are the ministers and dispensers of the divine bounty toward us. Accordingly, we are told how they watch for our safety, how they undertake our defence, direct our path, and take heed that no evil befall us...

Those again who limit the care of which God takes of each of us to a single angel, do great injury to themselves and to all the members of the Church, as if there were no value in those promises of auxiliary troops, who on every side encircling and defending us, embolden us to fight more manfully." Institutes of Christian Religion, Vol. I, 145, 147

It's not that Christians have one special "guardian" angel--it's that they have several.