Objection 1. It seems that there ought not to be seven sacraments. For the sacraments derive their efficacy from the Divine power, and the power of Christ's Passion. But the Divine power is one, and Christ's Passion is one; since "by one oblation He hath perfected for ever them that are sanctified" (Hebrews 10:14). Therefore there should be but one sacrament.
Objection 2. Further, a sacrament is intended as a remedy for the defect caused by sin. Now this is twofold, punishment and guilt. Therefore two sacraments would be enough.
Objection 3. Further, sacraments belong to the actions of the ecclesiastical hierarchy, as Dionysius explains (Eccl. Hier. v). But, as he says, there are three actions of the ecclesiastical hierarchy, namely, "to cleanse, to enlighten, to perfect." Therefore there should be no more than three sacraments.
Objection 4. Further, Augustine says (Contra Faust. xix) that the "sacraments" of the New Law are "less numerous" than those of the Old Law. But in the Old Law there was no sacrament corresponding to Confirmation and Extreme Unction. Therefore these should not be counted among the sacraments of the New Law.
Objection 5. Further, lust is not more grievous than other sins, as we have made clear in I-II, 74, 5; II-II, 154, 3. But there is no sacrament instituted as a remedy for other sins. Therefore neither should matrimony be instituted as a remedy for lust.
Objection 6. On the other hand, It seems that there should be more than seven sacraments. For sacraments are a kind of sacred sign. But in the Church there are many sanctifications by sensible signs, such as Holy Water the Consecration of Altars, and such like. Therefore there are more than seven sacraments.
Objection 7. Further, Hugh of St. Victor (De Sacram. i) says that the sacraments of the Old Law were oblations, tithes and sacrifices. But the Sacrifice of the Church is one sacrament, called the Eucharist. Therefore oblations also and tithes should be called sacraments.
Objection 8. Further, there are three kinds of sin, original, mortal and venial. Now Baptism is intended as a remedy against original sin, and Penance against mortal sin. Therefore besides the seven sacraments, there should be another against venial sin.
I answer that, As stated above (62, 5; 63, 1), the sacraments of the Church were instituted for a twofold purpose: namely, in order to perfect man in things pertaining to the worship of God according to the religion of Christian life, and to be a remedy against the defects caused by sin. And in either way it is becoming that there should be seven sacraments.
For spiritual life has a certain conformity with the life of the body: just as other corporeal things have a certain likeness to things spiritual. Now a man attains perfection in the corporeal life in two ways: first, in regard to his own person; secondly, in regard to the whole community of the society in which he lives, for man is by nature a social animal. With regard to himself man is perfected in the life of the body, in two ways; first, directly [per se, i.e. by acquiring some vital perfection; secondly, indirectly [per accidens, i.e. by the removal of hindrances to life, such as ailments, or the like. Now the life of the body is perfected "directly," in three ways. First, by generation whereby a man begins to be and to live: and corresponding to this in the spiritual life there is Baptism, which is a spiritual regeneration, according to Titus 3:5: "By the laver of regeneration," etc. Secondly, by growth whereby a man is brought to perfect size and strength: and corresponding to this in the spiritual life there is Confirmation, in which the Holy Ghost is given to strengthen us. Wherefore the disciples who were already baptized were bidden thus: "Stay you in the city till you be endued with power from on high" (Luke 24:49). Thirdly, by nourishment, whereby life and strength are preserved to man; and corresponding to this in the spiritual life there is the Eucharist. Wherefore it is said (John 6:54): "Except you eat of the flesh of the Son of Man, and drink His blood, you shall not have life in you."
And this would be enough for man if he had an impassible life, both corporally and spiritually; but since man is liable at times to both corporal and spiritual infirmity, i.e. sin, hence man needs a cure from his infirmity; which cure is twofold. one is the healing, that restores health: and corresponding to this in the spiritual life there is Penance, according to Psalm 40:5: "Heal my soul, for I have sinned against Thee." The other is the restoration of former vigor by means of suitable diet and exercise: and corresponding to this in the spiritual life there is Extreme Unction, which removes the remainder of sin, and prepares man for final glory. Wherefore it is written (James 5:15): "And if he be in sins they shall be forgiven him."
In regard to the whole community, man is perfected in two ways. First, by receiving power to rule the community and to exercise public acts: and corresponding to this in the spiritual life there is the sacrament of order, according to the saying of Hebrews 7:27, that priests offer sacrifices not for themselves only, but also for the people. Secondly in regard to natural propagation. This is accomplished by Matrimony both in the corporal and in the spiritual life: since it is not only a sacrament but also a function of nature.
We may likewise gather the number of the sacraments from their being instituted as a remedy against the defect caused by sin. For Baptism is intended as a remedy against the absence of spiritual life; Confirmation, against the infirmity of soul found in those of recent birth; the Eucharist, against the soul's proneness to sin; Penance, against actual sin committed after baptism; Extreme Unction, against the remainders of sins--of those sins, namely, which are not sufficiently removed by Penance, whether through negligence or through ignorance; order, against divisions in the community; Matrimony, as a remedy against concupiscence in the individual, and against the decrease in numbers that results from death.
Some, again, gather the number of sacraments from a certain adaptation to the virtues and to the defects and penal effects resulting from sin. They say that Baptism corresponds to Faith, and is ordained as a remedy against original sin; Extreme Unction, to Hope, being ordained against venial sin; the Eucharist, to Charity, being ordained against the penal effect which is malice. order, to Prudence, being ordained against ignorance; Penance to Justice, being ordained against mortal sin; Matrimony, to Temperance, being ordained against concupiscence; Confirmation, to Fortitude, being ordained against infirmity.
Reply to Objection 1. The same principal agent uses various instruments unto various effects, in accordance with the thing to be done. In the same way the Divine power and the Passion of Christ work in us through the various sacraments as through various instruments.
Reply to Objection 2. Guilt and punishment are diversified both according to species, inasmuch as there are various species of guilt and punishment, and according to men's various states and habitudes. And in this respect it was necessary to have a number of sacraments, as explained above.
Reply to Objection 3. In hierarchical actions we must consider the agents, the recipients and the actions. The agents are the ministers of the Church; and to these the sacrament of order belongs. The recipients are those who approach the sacraments: and these are brought into being by Matrimony. The actions are "cleansing," "enlightening," and "perfecting." Mere cleansing, however, cannot be a sacrament of the New Law, which confers grace: yet it belongs to certain sacramentals, i.e. catechism and exorcism. But cleansing coupled with enlightening, according to Dionysius, belongs to Baptism; and, for him who falls back into sin, they belong secondarily to Penance and Extreme Unction. And perfecting, as regards power, which is, as it were, a formal perfection, belongs to Confirmation: while, as regards the attainment of the end, it belongs to the Eucharist.
Reply to Objection 4. In the sacrament of Confirmation we receive the fulness of the Holy Ghost in order to be strengthened; while in Extreme Unction man is prepared for the immediate attainment of glory; and neither of these two purposes was becoming to the Old Testament. Consequently, nothing in the old Law could correspond to these sacraments. Nevertheless, the sacraments of the old Law were more numerous, on account of the various kinds of sacrifices and ceremonies.
Reply to Objection 5. There was need for a special sacrament to be applied as a remedy against venereal concupiscence: first because by this concupiscence, not only the person but also the nature is defiled: secondly, by reason of its vehemence whereby it clouds the reason.
Reply to Objection 6. Holy Water and other consecrated things are not called sacraments, because they do not produce the sacramental effect, which is the receiving of grace. They are, however, a kind of disposition to the sacraments: either by removing obstacles. thus holy water is ordained against the snares of the demons, and against venial sins: or by making things suitable for the conferring of a sacrament; thus the altar and vessels are consecrated through reverence for the Eucharist.
Reply to Objection 7. Oblations and tithes, both the Law of nature and in the Law of Moses, ere ordained not only for the sustenance of the ministers and the poor, but also figuratively; and consequently they were sacraments. But now they remain no longer as figures, and therefore they are not sacraments.
Reply to Objection 8. The infusion of grace is not necessary for the blotting out of venial sin. Wherefore, since grace is infused in each of the sacraments of the New Law, none of them was instituted directly against venial sin. This is taken away by certain sacramentals, for instance, Holy Water and such like. Some, however, hold that Extreme Unction is ordained against venial sin. But of this we shall speak in its proper place (XP, 30, 1).
Source: Summa Theologica (New Advent)