What is Schism?

"On the contrary, Augustine (Contra Faust. xx, 3; Contra Crescon. ii, 4) distinguishes between schism and heresy, for he says that a "schismatic is one who holds the same faith, and practises the same worship, as others, and takes pleasure in the mere disunion of the community, whereas a heretic is one who holds another faith from that of the Catholic Church." Therefore schism is not a generic sin. I answer that, As Isidore says (Etym. viii, 3), schism takes its name "from being a scission [lit. cut or divide] of minds," and scission is opposed to unity. Wherefore the sin of schism is one that is directly and essentially opposed to unity. For in the moral, as in the physical order, the species is not constituted by that which is accidental. Now, in the moral order, the essential is that which is intended, and that which results beside the intention, is, as it were, accidental. Hence the sin of schism is, properly speaking, a special sin, for the reason that the schismatic intends to sever himself from that unity 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."

[S.T., II Pt. of the II Pt., Q. 39, Art. 1]