Objection 1. It would seem that schismatics are not rightly punished with excommunication. For excommunication deprives a man chiefly of a share in the sacraments. But Augustine says (Contra Donat. vi, 5) that "Baptism can be received from a schismatic." Therefore it seems that excommunication is not a fitting punishment for schismatics.
Objection 2. Further, it is the duty of Christ's faithful to lead back those who have gone astray, wherefore it is written against certain persons (Ezekiel 34:4): "That which was driven away you have not brought again, neither have you sought that which was lost." Now schismatics are more easily brought back by such as may hold communion with them. Therefore it seems that they ought not to be excommunicated.
Objection 3. Further, a double punishment is not inflicted for one and the same sin, according to Nahum 1:9: "God will not judge the same twice" [Septuagint version]. Now some receive a temporal punishment for the sin of schism, according to23, 5, where it is stated: "Both divine and earthly laws have laid down that those who are severed from the unity of the Church, and disturb her peace, must be punished by the secular power." Therefore they ought not to be punished with excommunication.
On the contrary, It is written (Numbers 16:26): "Depart from the tents of these wicked men," those, to wit, who had caused the schism, "and touch nothing of theirs, lest you be involved in their sins."
I answer that, According to Wisdom 11:11, "By what things a man sinneth, by the same also he should be punished" [Vulgate: 'he is tormented']. Now a schismatic, as shown above (Article 1), commits a twofold sin: first by separating himself from communion with the members of the Church, and in this respect the fitting punishment for schismatics is that they be excommunicated. Secondly, they refuse submission to the head of the Church, wherefore, since they are unwilling to be controlled by the Church's spiritual power, it is just that they should be compelled by the secular power.
Reply to Objection 1. It is not lawful to receive Baptism from a schismatic, save in a case of necessity, since it is better for a man to quit this life, marked with the sign of Christ, no matter from whom he may receive it, whether from a Jew or a pagan, than deprived of that mark, which is bestowed in Baptism.
Reply to Objection 2. Excommunication does not forbid the intercourse whereby a person by salutary admonitions leads back to the unity of the Church those who are separated from her. Ondeed this very separation brings them back somewhat, because through confusion at their separation, they are sometimes led to do penance.
Reply to Objection 3. The punishments of the present life are medicinal, and therefore when one punishment does not suffice to compel a man, another is added: just as physicians employ several body medicines when one has no effect. On like manner the Church, when excommunication does not sufficiently restrain certain men, employs the compulsion of the secular arm. If, however, one punishment suffices, another should not be employed.
Source: Summa Theologica (New Advent)