Objection 1. It seems that the duties of piety towards one's parents should be omitted for the sake of religion. For Our Lord said (Luke 14:26): "If any man come to Me, and hate not his father, and mother, and wife, and children, and brethren, and sisters, yea and his own life also, he cannot be My disciple." Hence it is said in praise of James and John (Matthew 4:22) that they left "their nets and father, and followed" Christ. Again it is said in praise of the Levites (Deuteronomy 33:9): "Who hath said to his father, and to his mother: I do not know you; and to his brethren: I know you not; and their own children they have not known. These have kept Thy word." Now a man who knows not his parents and other kinsmen, or who even hates them, must needs omit the duties of piety. Therefore the duties of piety should be omitted for the sake of religion.
Objection 2. Further, it is written (Luke 9:59-60) that in answer to him who said: "Suffer me first to go and bury my father," Our Lord replied: "Let the dead bury their dead: but go thou, and preach the kingdom of God." Now the latter pertains to religion, while it is a duty of piety to bury one's father. Therefore a duty of piety should be omitted for the sake of religion.
Objection 3. Further, God is called "Our Father" by excellence. Now just as we worship our parents by paying them the duties of piety so do we worship God by religion. Therefore the duties of piety should be omitted for the sake of the worship of religion.
Objection 4. Further, religious are bound by a vow which they may not break to fulfil the observances of religion. Now in accordance with those observances they are hindered from supporting their parents, both on the score of poverty, since they have nothing of their own, and on the score of obedience, since they may not leave the cloister without the permission of their superior. Therefore the duties of piety towards one's parents should be omitted for the sake of religion.
On the contrary, Our Lord reproved the Pharisees (Matthew 15:3-6) who taught that for the sake of religion one ought to refrain from paying one's parents the honor we owe them.
I answer that, Religion and piety are two virtues. Now no virtue is opposed to another virtue, since according to the Philosopher, in his book on the Categories (Cap. De oppos.), "good is not opposed to good." Therefore it is impossible that religion and piety mutually hinder one another, so that the act of one be excluded by the act of the other. Now, as stated above (I-II, 07, 2; I-II, 18, 3), the act of every virtue is limited by the circumstances due thereto, and if it overstep them it will be an act no longer of virtue but of vice. Hence it belongs to piety to pay duty and homage to one's parents according to the due mode. But it is not the due mode that man should tend to worship his father rather than God, but, as Ambrose says on Luke 12:52, "the piety of divine religion takes precedence of the claims of kindred."
Accordingly, if the worship of one's parents take one away from the worship of God it would no longer be an act of piety to pay worship to one's parents to the prejudice of God. Hence Jerome says (Ep. ad Heliod.): "Though thou trample upon thy father, though thou spurn thy mother, turn not aside, but with dry eyes hasten to the standard of the cross; it is the highest degree of piety to be cruel in this matter." Therefore in such a case the duties of piety towards one's parents should be omitted for the sake of the worship religion gives to God. If, however, by paying the services due to our parents, we are not withdrawn from the service of God, then will it be an act of piety, and there will be no need to set piety aside for the sake of religion.
Reply to Objection 1. Gregory expounding this saying of our Lord says (Hom. xxxvii in Ev.) that "when we find our parents to be a hindrance in our way to God, we must ignore them by hating and fleeing from them." For if our parents incite us to sin, and withdraw us from the service of God, we must, as regards this point, abandon and hate them. It is in this sense that the Levites are said to have not known their kindred, because they obeyed the Lord's command, and spared not the idolaters (Exodus 32). James and John are praised for leaving their parents and following our Lord, not that their father incited them to evil, but because they deemed it possible for him to find another means of livelihood, if they followed Christ.
Reply to Objection 2. Our Lord forbade the disciple to bury his father because, according to Chrysostom (Hom. xxviii in Matth.), "Our Lord by so doing saved him from many evils, such as the sorrows and worries and other things that one anticipates under these circumstances. For after the burial the will had to be read, the estate had to be divided, and so forth: but chiefly, because there were others who could see to the funeral." Or, according to Cyril's commentary on Luke 9, "this disciple's request was, not that he might bury a dead father, but that he might support a yet living father in the latter's old age, until at length he should bury him. This is what Our Lord did not grant, because there were others, bound by the duties of kindred, to take care of him."
Reply to Objection 3. Whatever we give our parents out of piety is referred by us to God; just as other works of mercy which we perform with regard to any of our neighbors are offered to God, according to Matthew 25:40: "As long as you did it to one of... My least... you did it to Me." Accordingly, if our carnal parents stand in need of our assistance, so that they have no other means of support, provided they incite us to nothing against God, we must not abandon them for the sake of religion. But if we cannot devote ourselves to their service without sin, or if they can be supported without our assistance, it is lawful to forego their service, so as to give more time to religion.
Reply to Objection 4. We must speak differently of one who is yet in the world, and of one who has made his profession in religion. For he that is in the world, if he has parents unable to find support without him, he must not leave them and enter religion, because he would be breaking the commandment prescribing the honoring of parents. Some say, however, that even then he might abandon them, and leave them in God's care. But this, considered aright, would be to tempt God: since, while having human means at hand, he would be exposing his parents to danger, in the hope of God's assistance. on the other hand, if the parents can find means of livelihood without him, it is lawful for him to abandon them and enter religion, because children are not bound to support their parents except in cases of necessity, as stated above. He that has already made his profession in religion is deemed to be already dead to the world: wherefore he ought not, under pretext of supporting his parents, to leave the cloister where he is buried with Christ, and busy himself once more with worldly affairs. Nevertheless he is bound, saving his obedience to his superiors, and his religious state withal, to make points efforts for his parents' support.
Source: Summa Theologica (New Advent)