Objection 1. It would seem that spiritual relationship is contracted by Baptism only. For as bodily kinship is to bodily birth, so is spiritual kinship to spiritual birth. Now Baptism alone is called spiritual birth. Therefore spiritual kinship is contracted by Baptism only, even as only by carnal birth is carnal kinship contracted.
Objection 2. Further, a character is imprinted in order as in Confirmation. But spiritual relationship does not result from receiving orders. Therefore it does not result from Confirmation but only from Baptism.
Objection 3. Further, sacraments are more excellent than sacramentals. Now spiritual relationship does not result from certain sacraments, for instance from Extreme Unction. Much less therefore does it result from catechizing, as some maintain.
Objection 4. Further, many other sacramentals are attached to Baptism besides catechizing. Therefore spiritual relationship is not contracted from catechism any more than from the others.
Objection 5. Further, prayer is no less efficacious than instruction of catechism for advancement in good. But spiritual relationship does not result from prayer. Therefore it does not result from catechism.
Objection 6. Further, the instruction given to the baptized by preaching to them avails no less than preaching to those who are not yet baptized. But no spiritual relationship results from preaching. Neither therefore does it result from catechism.
Objection 7. On the other hand, It is written (1 Corinthians 4:15): "In Christ Jesus by the gospel I have begotten you." Now spiritual birth causes spiritual relationship. Therefore spiritual relationship results from the preaching of the gospel and instruction, and not only from Baptism.
Objection 8. Further, as original sin is taken away by Baptism, so is actual sin taken away by Penance. Therefore just as Baptism causes spiritual relationship, so also does Penance.
Objection 9. Further, "father" denotes relationship. Now a man is called another's spiritual father in respect of Penance, teaching, pastoral care and many other like things. Therefore spiritual relationship is contracted from many other sources besides Baptism and Confirmation.
I answer that, There are three opinions on this question. Some say that as spiritual regeneration is bestowed by the sevenfold grace of the Holy Ghost, it is caused by means of seven things, beginning with the first taste of blessed salt and ending with Confirmation given by the bishop: and they say that spiritual relationship is contracted by each of these seven things. But this does not seem reasonable, for carnal relationship is not contracted except by a perfect act of generation. Wherefore affinity is not contracted except there be mingling of seeds, from which it is possible for carnal generation to follow. Now spiritual generation is not perfected except by a sacrament: wherefore it does not seem fitting for spiritual relationship to be contracted otherwise than through a sacrament. Hence others say that spiritual relationship is only contracted through three sacraments, namely catechism, Baptism and Confirmation, but these do not apparently know the meaning of what they say, since catechism is not a sacrament but a sacramental. Wherefore others say that it is contracted through two sacraments only, namely Confirmation and Baptism, and this is the more common opinion. Some however of these say that catechism is a weak impediment, since it hinders the contracting of marriage but does not void the contract.
Reply to Objection 1. Carnal birth is twofold. The first is in the womb, wherein that which is born is a weakling and cannot come forth without danger: and to this birth regeneration by Baptism is likened; wherein a man is regenerated as though yet needing to be fostered in the womb of the Church. The second is birth from out of the womb, when that which was born in the womb is so far strengthened that it can without danger face the outer world which has a natural corruptive tendency. To this is likened Confirmation, whereby man being strengthened goes forth abroad to confess the name of Christ. Hence spiritual relationship is fittingly contracted through both these sacraments.
Reply to Objection 2. The effect of the sacrament of order is not regeneration but the bestowal of power, for which reason it is not conferred on women, and consequently no impediment to marriage can arise therefrom. Hence this kind of relationship does not count.
Reply to Objection 3. In catechism one makes a profession of future Baptism, just as in betrothal one enters an engagement of future marriage. Wherefore just as in betrothal a certain kind of propinquity is contracted, so is there in catechism, whereby marriage is rendered at least unlawful, as some say; but not in the other sacraments.
Reply to Objection 4. There is not made a profession of faith in the other sacramentals of Baptism, as in catechism: wherefore the comparison fails. The same answer applies to the Fifth and Sixth Objections.
Reply to Objection 7. The Apostle had instructed them in the faith by a kind of catechism; and consequently his instruction was directed to their spiritual birth.
Reply to Objection 8. Properly speaking a spiritual relationship is not contracted through the sacrament of Penance. Wherefore a priest's son can marry a woman whose confession the priest has heard, else in the whole parish he could not find a woman whom he could marry. Nor does it matter that by Penance actual sin is taken away, for this is not a kind of birth, but a kind of healing. Nevertheless Penance occasions a kind of bond between the woman penitent and the priest, that has a resemblance to spiritual relationship, so that if he have carnal intercourse with her, he sins as grievously as if she were his spiritual daughter. The reason of this is that the relations between priest and penitent are most intimate, and consequently in order to remove the occasion of sin this prohibition [Can. Omnes quos, and seqq., Caus. xxx] was made.
Reply to Objection 9. A spiritual father is so called from his likeness to a carnal father. Now as the Philosopher says (Ethic. viii, 2) a carnal father gives his child three things, being nourishment and instruction: and consequently a person's spiritual father is so called from one of these three things. Nevertheless he has not, through being his spiritual father, a spiritual relationship with him, unless he is like a (carnal) father as to generation which is the way to being. This solution may also be applied to the foregoing Eighth Objection.
Source: Summa Theologica (New Advent)