"INASMUCH, I repeat, as this is the case, we believe also in The Holy Church, [intending thereby] assuredly the catholic. For both heretics and schismatics style their congregations churches. But heretics, in holding false opinions regarding God, do injury to the faith itself; while schismatics, on the other hand, in wicked separations break off from brotherly charity, although they may believe just what we believe. Wherefore neither do the heretics belong to the Church catholic, which loves God; nor do the schismatics form a part of the same, inasmuch as it loves the neighbor, and consequently readily forgives the neighbor’s sins, because it prays that forgiveness may be extended to itself by Him who has reconciled us to Himself, doing away with all past things, and calling us to a new life. And until we reach the perfection of this new life, we cannot be without sins. Nevertheless it is a matter of consequence of what sort those sins may be."
~St. Augustine: Treatise on Faith and the Creed (De Fide et Symbolo), 21 (Ch. 10).