Objection 1. It would seem that a religious does not sin more grievously than a secular by the same kind of sin. For it is written (2 Chronicles 30:18-19): "The Lord Who is good will show mercy to all them who with their whole heart seek the Lord the God of their fathers, and will not impute it to them that they are not sanctified." Now religious apparently follow the Lord the God of their fathers with their whole heart rather than seculars, who partly give themselves and their possessions to God and reserve part for themselves, as Gregory says (Hom. xx in Ezech.). Therefore it would seem that it is less imputed to them if they fall short somewhat of their sanctification.
Objection 2. Further, God is less angered at a man's sins if he does some good deeds, according to (2 Chronicles 19:2),3, "Thou helpest the ungodly, and thou art joined in friendship with them that hate the Lord, and therefore thou didst deserve indeed the wrath of the Lord: but good works are found in thee." Now religious do more good works than seculars. Therefore if they commit any sins, God is less angry with them.
Objection 3. Further, this present life is not carried through without sin, according to (James 3:2), "In many things we all offend." Therefore if the sins of religious were more grievous than those of seculars it would follow that religious are worse off than seculars: and consequently it would not be a wholesome counsel to enter religion.
On the contrary, The greater the evil the more it would seem to be deplored. But seemingly the sins of those who are in the state of holiness and perfection are the most deplorable, for it is written (Jeremiah 23:9): "My heart is broken within me," and afterwards (Jeremiah 23:11): "For the prophet and the priest are defiled; and in My house I have found their wickedness." Therefore religious and others who are in the state of perfection, other things being equal, sin more grievously.
I answer that, A sin committed by a religious may be in three ways more grievous than a like sin committed by a secular. First, if it be against his religious vow; for instance if he be guilty of fornication or theft, because by fornication he acts against the vow of continence, and by theft against the vow of poverty; and not merely against a precept of the divine law. Secondly, if he sin out of contempt, because thereby he would seem to be the more ungrateful for the divine favors which have raised him to the state of perfection. Thus the Apostle says (Hebrews 10:29) that the believer "deserveth worse punishments" who through contempt tramples under foot the Son of God. Hence the Lord complains (Jeremiah 11:15): "What is the meaning that My beloved hath wrought much wickedness in My house?" Thirdly, the sin of a religious may be greater on account of scandal, because many take note of his manner of life: wherefore it is written (Jeremiah 23:14): "I have seen the likeness of adulterers, and the way of lying in the Prophets of Jerusalem; and they strengthened the hands of the wicked, that no man should return from his evil doings."
On the other hand, if a religious, not out of contempt, but out of weakness or ignorance, commit a sin that is not against the vow of his profession, without giving scandal (for instance if he commit it in secret) he sins less grievously in the same kind of sin than a secular, because his sin if slight is absorbed as it were by his many good works, and if it be mortal, he more easily recovers from it. First, because he has a right intention towards God, and though it be intercepted for the moment, it is easily restored to its former object. Hence Origen commenting on Psalm 36:24, "When he shall fall he shall not be bruised," says (Hom. iv in Ps. 36): "The wicked man, if he sin, repents not, and fails to make amends for his sin. But the just man knows how to make amends and recover himself; even as he who had said: 'I know not the man,' shortly afterwards when the Lord had looked on him, knew to shed most bitter tears, and he who from the roof had seen a woman and desired her knew to say: 'I have sinned and done evil before Thee.'" Secondly, he is assisted by his fellow-religious to rise again, according to Ecclesiastes 4:10, "If one fall he shall be supported by the other: woe to him that is alone, for when he falleth he hath none to lift him up."
Reply to Objection 1. The words quoted refer to things done through weakness or ignorance, but not to those that are done out of contempt.
Reply to Objection 2. Josaphat also, to whom these words were addressed, sinned not out of contempt, but out of a certain weakness of human affection.
Reply to Objection 3. The just sin not easily out of contempt; but sometimes they fall into a sin through ignorance or weakness from which they easily arise. If, however, they go so far as to sin out of contempt, they become most wicked and incorrigible, according to the word of Jeremiah 2:20: "Thou hast broken My yoke, thou hast burst My bands, and thou hast said: 'I will not serve.' For on every high hill and under every green tree thou didst prostitute thyself." Hence Augustine says (Ep. lxxviii ad Pleb. Hippon.): "From the time I began to serve God, even as I scarcely found better men than those who made progress in monasteries, so have I not found worse than those who in the monastery have fallen."
Source: Summa Theologica (New Advent)