HKB
which is the effect of charity: because charity unites not only one person to another with the bond of spiritual love, but also the whole Church in unity of spirit.
Accordingly schismatics properly so called are those who, wilfully and intentionally separate themselves from the unity of the Church; for this is the chief unity, and the particular unity of several individuals among themselves is subordinate to the unity of the Church, even as the mutual adaptation of each member of a natural body is subordinate to the unity of the whole body. Now the unity of the Church consists in two things; namely, in the mutual connection or communion of the members of the Church, and again in the subordination of all the members of the Church to the one head, according to Col. 2:18, 19: "Puffed up by the sense of his flesh, and not holding the Head, from which the whole body, by joints and bands, being supplied with nourishment and compacted, groweth unto the increase of God." Now this Head is Christ Himself, Whose viceregent in the Church is the Sovereign Pontiff. Wherefore schismatics are those who refuse to submit to the Sovereign Pontiff, and to hold communion with those members of the Church who acknowledge his supremacy.
Reply Obj. 1: The division between man and God that results from sin is not intended by the sinner: it happens beside his intention as a result of his turning inordinately to a mutable good, and so it is not schism properly so called.
Reply Obj. 2: The essence of schism consists in rebelliously disobeying the commandments: and I say "rebelliously," since a schismatic both obstinately scorns the commandments of the Church, and refuses to submit to her judgment. But every sinner does not do this, wherefore not every sin is a schism.
Reply Obj. 3: Heresy and schism are distinguished in respect of those things to which each is opposed essentially and directly. For heresy is essentially opposed to faith, while schism is essentially opposed to the unity of ecclesiastical charity. Wherefore just as faith and charity are different virtues, although whoever lacks faith lacks charity, so too schism and heresy are different vices, although whoever is a heretic is also a schismatic, but not conversely. This is what Jerome says in his commentary on the Epistle to the Galatians [*In Ep. ad Tit. iii, 10]: "I consider the difference between schism and heresy to be that heresy holds false doctrine while schism severs a man from the Church." Nevertheless, just as the loss of charity is the road to the loss of faith, according to 1 Tim. 1:6: "From which things," i.e. charity and the like, "some going astray, are turned aside into vain babbling," so too, schism is the road to heresy. Wherefore Jerome adds (In Ep. ad Tit. iii, 10) that "at the outset it is possible, in a certain respect, to find a difference between schism and heresy: yet there is no schism that does not devise some heresy for itself, that it may appear to have had a reason for separating from the Church."
SECOND ARTICLE [II-II, Q. 39, Art. 2]
Whether Schism Is a Graver Sin Than Unbelief?