Now, we grant that by the sacraments men are restored to grace; nonetheless, they are not immediately restored to immortality. We have given the reason for this. But things which are corruptible cannot be perpetuated except by generation. Since, then, the people of the faithful had to be perpetuated unto the end of the world, this had to be done by generation, by which, also, the human species is perpetuated.
 But let us consider this: When something is ordered to different ends there must be differing principles directing it to the end, for the end is proportioned to the agent. Human generation, of course, is ordered to many things; namely, to the perpetuity of the species and to the perpetuity of some political good—the perpetuity of a people in some state for example. It is also ordered to the perpetuity of the Church, which consists in the collection of the faithful. Accordingly, generation of this kind must be subject to a diversity of directions. Therefore, so far as it is ordered to the good of nature, which is the perpetuity of the species, it is directed to the end by nature inclining to this end; thus, one calls it a duty of nature. But, so far as generation is ordered to a political good, it is subject to the ordering of civil law. Then, so far as it is ordered to the good of the Church, it must be subject to the government of the Church. But things which are dispensed to the people by the ministers of the Church are called sacraments. Matrimony, then, in that it consists in the union of a husband and wife purposing to generate and educate offspring for the worship of God, is a sacrament of the Church; hence, also, a certain blessing on those marrying is given by the ministers of the Church.
13] And as in the other sacraments by the thing done outwardly a sign is made of a spiritual thing, so, too, in this sacrament by the union of husband and wife a sign of the union of Christ and the Church is made; in the Apostle’s words: “This is a great sacrament, but I speak in Christ and in the church” (Eph. 5:32).
 And because the sacraments effect that of which they are made signs, one must believe that in this sacrament a grace is conferred on those marrying, and that by this grace they are included in the union of Christ and the Church, which is most especially necessary to them, that in this way in fleshly and earthly things they may purpose not to be disunited from Christ and the Church.
 Since, then, the union of husband and wife gives a sign of the union of Christ and the Church, that which makes the sign must correspond to that whose sign it is. Now, the union of Christ and the Church is a union of one to one to be held forever. For there is one Church, as the Canticle (6:8) says: “One is My dove, My perfect one.” And Christ will never be separated from His Church, for He Himself says: “Behold I am with you all days even to the consummation of the world” (Mat. 28:20); and, further: “we shall be always with the Lord” (1 Thes. 4:16), as the Apostle says. Necessarily, then, matrimony as a sacrament of the Church is a union of one man to one woman to be held indivisibly, and this is included in the faithfulness by which the man and wife are bound to one another.
 Thus, then, there are three goods of matrimony as a sacrament of the Church: namely, offspring to be accepted and educated for the worship of God; fidelity by which one man is bound to one wife; and the sacrament—and, in accord with this—there is indivisibility in the marriage union, in so far as it is a sacrament of the union of Christ and the Church.
 Now, all the other things one ought to consider in matrimony we have dealt with in Book III.
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