Summa summae S. Thomae, Tomus III

Apostasia in Greek means the same as discessio or desertio, departure or desertion, in Latin. It is taken here for a withdrawal from a good which one has solemnly professed. And apostasy is threefold: from the faith, from religion, and from order¹

I say 1°: Apostasy from the faith is the desertion of the whole of the faith. And by this it is distinguished from heresy, which is a desertion of the faith as to a part only.

[Nor indeed does the crime of apostasy require the profession of some false religion. Hence under the name of apostates come those who vindicate to themselves the liberty of thinking and of admitting whatever pleases them, by that very fact rejecting Catholic doctrine: among whom must be numbered rationalists, deists, naturalists, pantheists, spiritists, and in general all who, after receiving baptism, have withdrawn entirely from the Christian faith, even if they have not passed over to Protestantism, or Judaism, or Mohammedanism, or paganism.

From what has been said it is clear that apostasy does not differ from heresy essentially, but only according to more and less, which does not diversify the species. For, as the Angelic Doctor says, this q. 12, a. 1, ad 3, « apostasy looks to infidelity as the term to which the movement of one withdrawing from the faith tends. Hence apostasy does not import a determinate species of infidelity, but a certain circumstance aggravating it, according to that of 2 Pet., II, 21: It had been better for them not to have known the way of truth, than, after they have known it, to turn back. »

Apostates are bound by the same excommunication as heretics, as was related above, de Poenis haereticorum.]

I say 2°: He who abandons an approved religion which he has professed tacitly or expressly, without the legitimate license of his superiors, with the intention of not returning, even if he does not lay aside the habit, is an apostate²; because, as the habit does not make the monk, but the profession of the religious life, so the laying aside of the habit does not make the apostate, but the withdrawal from the religious life. Those who laid aside the habit were formerly excommunicated by that very fact, by the chapter Ut periculosa, tit. Ne clerici et monachi, in 6°, lib. III. But he who does not lay aside the habit is not excommunicated by the common law; each one, however, ought to consult the laws of his own Order. In our Order of Preachers he is excommunicated by that very fact, dist. I, cap. 20, litt. A. Likewise, if he rashly lays aside the habit, that is, with the mind of wandering about, even though with the will of returning, he is indeed not an apostate, yet he is excommunicated by that very fact by the cited chapter, Ut periculosa.

But if he lays aside the habit not rashly and for the sake of wandering about, but for a legitimate cause, either within the cloister, for instance for the sake of sleep, rest, infirmity, etc., or outside the cloister, for instance for fear of death while traveling through the lands of the infidels, or for the sake of avoiding scandal while traveling by night, or for some other reasonable cause, he does not sin, nor does he incur excommunication; because the celebrated canon is directed only against the rash laying aside of the habit, whereby there would be matter apt for wandering about.

[By the constitution Apostolicae Sedis, « regulars or nuns who presume to contract marriage after a solemn vow of chastity » incur excommunication latae sententiae reserved to their Ordinaries, that is, their major superiors. This crime includes not only a violation of the vow of chastity, but also involves apostasy from the religious life. — It must be observed that this censure was decreed only against those solemnly professed.]

I say 3°: He who withdraws from sacred orders back to the secular state without legitimate dispensation is an apostate and sins mortally; because he abandons the state to which he had been solemnly bound over by the Church, and which many canons forbid him to abandon, penalties being imposed against transgressors.

[The aforesaid constitution, with excommunication latae sententiae reserved to bishops or ordinaries, punishes « clerics constituted in sacred orders who presume to contract marriage ». For such men despise more than the law of chastity, and are held as apostates. And it suffices for incurring this censure that the attempt at marriage be made before a civil magistrate, in places where the Tridentine decree on clandestinity is in force. So pronounced the S. Congregation of the Holy Office on December 22, 1880.]