Thomas Aquinas

“ In the vow of religious profession it is not the spiritual marriage itself that is expressed in words which refer to the future, but an act of the spiritual marriage, namely obedience or observance of the rule. If, however, a man vow spiritual marriage in the future, it is not a spiritual marriage, for a man does not become a monk by taking such a vow, but promises to become one. ”

“ Wherefore, since in matrimony there is a kind of spiritual joining together, in so far as matrimony is a sacrament, and a certain material joining together, in so far as it is directed to an office of nature and of civil life, it follows that the spiritual joining is the effect of the Divine power by means of the material joining. Therefore seeing that the joinings of material contracts are effected by mutual consent, it follows that the joining together of marriage is effected in the same way. ”

“ Even in other contracts, a man who uses words referring to the future, does not transfer the power over his property to another person---for instance if he were to say "I will give thee"---but only when he uses words indicative of the present. ”

“ Further, the mental consent of one person cannot be known to another, except in so far as it is expressed in words. If then the expression of the words is not enough, and inward consent is required in both parties, neither of them will be able to know that he is truly married to the other; and consequently whenever he uses marriage he will commit fornication. ”

“ And since material contracts are not feasible unless the contracting parties express their will to one another in words, it follows that the consent which makes a marriage must also be expressed in words, so that the expression of words is to marriage what the outward washing is to Baptism. ”

“ In such a case the Church compels him to hold to his first wife, because the Church judges according to outward appearances; nor is she deceived in justice or right, although she is deceived in the facts of the case. ”