Thomas Aquinas

“ Although adultery is opposed to marriage as fulfilling an office of nature, more directly than unbelief, it is the other way about if we consider marriage as a sacrament of the Church, from which source it derives perfect stability, inasmuch as it signifies the indissoluble union of Christ with the Church. Wherefore the marriage that is not ratified can be dissolved as to the marriage tie on account of unbelief rather than on account of adultery. ”

“ Disparity of worship makes a person simply unfit for lawful marriage, whereas the condition of bondage does not, but only where it is unknown. Hence there is no comparison between an unbeliever and a bondswoman. ”

“ Further, every impediment to marriage is in some way contrary to marriage. But unbelief is not contrary to marriage, since marriage fulfills an office of nature whose dictate faith surpasses. Therefore disparity of worship is not an impediment to marriage. ”

“ To divorce a wife is contrary to the law of nature, wherefore it is not lawful for an unbeliever to divorce his wife. Hence if he be converted after divorcing one and marrying another, the same judgment is to be pronounced in this case as in the case of a man who had several wives, because if he wish to be converted he is bound to take the first whom he had divorced and to put the other away.
Art. 4 - Whether a believer can, after his conversion, put away his unbelieving wife if she be willing to cohabit with him without insult to the Creator? ”

“ For if the unbeliever be willing to cohabit without insult to the Creator---that is without drawing the other to unbelief---the believer is free to part from the other, but by parting is not permitted to marry again. But if the unbeliever refuse to cohabit without insult to the Creator, by making use of blasphemous words and refusing to hear Christ's name, then if she strive to draw him to unbelief, the believing husband after parting from her may be united to another in marriage. ”

“ Wherefore if the husband leave her after he has changed his life by dying to his former life, this is nowise prejudicial to her. Now he who goes over to the religious life dies but a spiritual death and not a bodily death. Wherefore if the marriage be consummated, the husband cannot enter religion without his wife's consent, whereas he can before carnal connection when there is only a spiritual connection. ”

“ Further, an adulterous husband cannot put away an adulterous wife, even after he has done penance for his adultery. Therefore if an adulterous and an unbelieving husband are to be judged alike, neither can the believer put aside the unbeliever, even after his conversion to the faith. ”

“ Further, a marriage contracted between believers is more perfect than one contracted between unbelievers. Now, if believers marry within the degrees forbidden by the Church, their marriage is void. Therefore the same applies to unbelievers, and thus a believing husband cannot remain with an unbelieving wife, at any rate, if as an unbeliever he married her within the forbidden degrees. ”

“ Consequently, since those things which are for the sake of the end are proportionate to the end, the marriage that tends to the first perfection is imperfect and material in comparison with that which tends to the second perfection. And since the first perfection can be common to unbelievers and believers, while the second belongs only to believers, it follows that between unbelievers there is marriage indeed, but not perfected by its ultimate perfection as there is between believers. ”

“ Further, the removal of what comes after does not imply the removal of what comes first. Now marriage belongs to an office of nature, which precedes the state of grace, the principle of which is faith. ”

“ The husband ought not to take a vow of continence nor enter into a second marriage, if after his conversion there be a reasonable hope of the conversion of his wife, because the wife's conversion would be more difficult if she knew she was deprived of her husband. ”

“ The primal union of the soul to God is by faith, and consequently the soul is thereby espoused to God as it were, according to Osee 2:20, "I will espouse thee to Me in faith." Hence in Holy Writ idolatry and unbelief are specially designated by the name of fornication: whereas other sins are called spiritual fornications by a more remote signification. ”

“ Either the child has reached a perfect age, and then it is free to follow either the believing father or the unbelieving mother, or else it is under age, and then it should be given to the believer notwithstanding that it needs the mother's care for its education. ”

“ Further, spiritual adultery is more grievous than carnal. But a man can put his wife away, as to cohabitation, on account of carnal adultery. Much more, therefore, can he do so on account of unbelief, which is spiritual adultery. ”

“ The chastity and other virtues of unbelievers are said not to be real, because they cannot attain the end of real virtue, which is real happiness. Thus we say it is not a real wine if it has not the effect of wine. ”

“ I answer that, Different things are competent and expedient to man according as his life is of one kind or of another. Wherefore he who dies to his former life is not bound to those things to which he was bound in his former life. ”

“ They are not, however, forbidden by Divine law, because before God, however much one may stray from the faith, this makes no difference to one's being removed from grace: nor is it forbidden by any law of the Church who has not to judge of those who are without. ”

“ Further, husband and wife are equal in the marriage tie. Since, then, it is unlawful for the unbelieving wife to marry again while her husband lives, it would seem that neither can the believing husband do so. ”