Ignatius was using the word to mean "universal," i.e., accepting all the teachings of Christianity, not picking and choosing which parts to believe. Even early Christians were picking and choosing, i.e., Jesus is fully God but not fully man, or vice versa, and all the other early heresies.
One entry found for heresy.
[Folks were taking and choosing a forming sects right at the start, and the reformation was simply more of the same old picking and choosing.]
1 a : adherence to a religious opinion contrary to church dogma b : denial of a revealed truth by a baptized member of the Roman Catholic Church c : an opinion or doctrine contrary to church dogma
2 a : dissent or deviation from a dominant theory, opinion, or practice b : an opinion, doctrine, or practice contrary to the truth or to generally accepted beliefs or standards
So even in Ignatius' time it became necessary to delineate the "pickers and choosers", i.e., heretics, from those Christians who accepted the entire, universal, or catholic gospel.
And as noted above, to be a real Christian in early Christian times meant to believe in the Real Presence, in a hierachy, and in the authority of men, i.e., the bishop, to lose and to bind.