Objection 1. It would seem that the "sacrament" is not the chief of the marriage goods. For the end is principal in everything. Now the end of marriage is the offspring. Therefore the offspring is the chief marriage good.
Objection 2. Further, in the specific nature the difference is more important than the genus, even as the form is more important than matter in the composition of a natural thing. Now "sacrament" refers to marriage on the part of its genus, while "offspring" and "faith" refer thereto on the part of the difference whereby it is a special kind of sacrament. Therefore these other two are more important than sacrament in reference to marriage.
Objection 3. Further, just as we find marriage without "offspring" and without "faith," so do we find it without indissolubility, as in the case where one of the parties enters religion before the marriage is consummated. Therefore neither from this point of view is "sacrament" the most important marriage good.
Objection 4. Further, an effect cannot be more important than its cause. Now consent, which is the cause of matrimony, is often changed. Therefore the marriage also can be dissolved and consequently inseparability is not always a condition of marriage.
Objection 5. Further, the sacraments which produce an everlasting effect imprint a character. But no character is imprinted in matrimony. Therefore it is not conditioned by a lasting inseparability. Consequently just as there is marriage without "offspring" so is there marriage without "sacrament," and thus the same conclusion follows as above.
On the contrary, That which has a place in the definition of a thing is most essential thereto. Now inseparability, which pertains to sacrament, is placed in the definition of marriage (44, 3), while offspring and faith are not. Therefore among the other goods sacrament is the most essential to matrimony.
Further, the Divine power which works in the sacraments is more efficacious than human power. But "offspring" and "faith" pertain to matrimony as directed to an office of human nature, whereas "sacrament" pertains to it as instituted by God. Therefore sacrament takes a more important part in marriage than the other two.
I answer that, This or that may be more important to a thing in two ways, either because it is more essential or because it is more excellent. If the reason is because it is more excellent, then "sacrament" is in every way the most important of the three marriage goods, since it belongs to marriage considered as a sacrament of grace; while the other two belong to it as an office of nature; and a perfection of grace is more excellent than a perfection of nature. If, however, it is said to be more important because it is more essential, we must draw a distinction; for "faith" and "offspring" can be considered in two ways. First, in themselves, and thus they regard the use of matrimony in begetting children and observing the marriage compact; while inseparability, which is denoted by "sacrament," regards the very sacrament considered in itself, since from the very fact that by the marriage compact man and wife give to one another power the one over the other in perpetuity, it follows that they cannot be put asunder. Hence there is no matrimony without inseparability, whereas there is matrimony without "faith" and "offspring," because the existence of a thing does not depend on its use; and in this sense "sacrament" is more essential to matrimony than "faith" and "offspring." Secondly, "faith" and "offspring" may be considered as in their principles, so that "offspring" denote the intention of having children, and "faith" the duty of remaining faithful, and there can be no matrimony without these also, since they are caused in matrimony by the marriage compact itself, so that if anything contrary to these were expressed in the consent which makes a marriage, the marriage would be invalid. Taking "faith" and "offspring" in this sense, it is clear that "offspring" is the most essential thing in marriage, secondly "faith," and thirdly "sacrament"; even as to man it is more essential to be in nature than to be in grace, although it is more excellent to be in grace.
Reply to Objection 1. The end as regards the intention stands first in a thing, but as regards the attainment it stands last. It is the same with "offspring" among the marriage goods; wherefore in a way it is the most important and in another way it is not.
Reply to Objection 2. Sacrament, even as holding the third place among the marriage goods, belongs to matrimony by reason of its difference; for it is called "sacrament" from its signification of that particular sacred thing which matrimony signifies.
Reply to Objection 3. According to Augustine (De Bono Conjug. ix), marriage is a good of mortals, wherefore in the resurrection "they shall neither marry nor be married" (Matthew 22:30). Hence the marriage bond does not last after the life wherein it is contracted, and consequently it is said to be inseparable, because it cannot be sundered in this life, but either by bodily death after carnal union, or by spiritual death after a merely spiritual union.
Reply to Objection 4. Although the consent which makes a marriage is not everlasting materially, i.e. in regard to the substance of the act, since that act ceases and a contrary act may succeed it, nevertheless formally speaking it is everlasting, because it is a consent to an ever lasting bond, else it would not make a marriage, for a consent to take a woman for a time makes no marriage. Hence it is everlasting formally, inasmuch as an act takes its species from its object; and thus it is that matrimony derives its inseparability from the consent.
Reply to Objection 5. In those sacraments wherein a character is imprinted, power is given to perform spiritual actions; but in matrimony, to perform bodily actions. Wherefore matrimony by reason of the power which man and wife receive over one another agrees with the sacraments in which a character is imprinted, and from this it derives its inseparability, as the Master says (Sent. iv, D, 31); yet it differs from them in so far as that power regards bodily acts; hence it does not confer a spiritual character.
Source: Summa Theologica (New Advent)