Objection 1. It would seem that error should not be reckoned in itself an impediment to marriage. For consent, which is the efficient cause of marriage, is hindered in the same way as the voluntary. Now the voluntary, according to the Philosopher (Ethic. iii, 1), may be hindered by ignorance. But ignorance is not the same as error, because ignorance excludes knowledge altogether, whereas error does not, since "error is to approve the false as though it were true," according to Augustine (De Trin. ix, 11). Therefore ignorance rather than error should have been reckoned here as an impediment to marriage.
Objection 2. Further, that which of its very nature can be an impediment to marriage is in opposition to the good of marriage. But error is not a thing of this kind. Therefore error is not by its very nature an impediment to marriage.
Objection 3. Further, just as consent is required for marriage, so is intention required for baptism. Now if one were to baptize John, thinking to baptize Peter, John would be baptized none the less. Therefore error does not annul matrimony.
Objection 4. Further, there was true marriage between Lia and Jacob, and yet, in this case, there was error. Therefore error does not annul a marriage.
On the contrary, It is said in the Digests (Si per errorem, ff. De jurisdic. omn. judic.): "What is more opposed to consent than error?" Now consent is required for marriage. Therefore error is an impediment to matrimony.
Further, consent denotes something voluntary. Now error is an obstacle to the voluntary, since "the voluntary," according to the Philosopher (Ethic. iii, 1), Damascene (De Fide Orth. ii, 24), and Gregory of Nyssa [Nemesius] (De Nat. Hom. xxxii), "is that which has its principle in one who has knowledge of singulars which are the matter of actions." But this does not apply to one who is in error. Therefore error is an impediment to matrimony.
I answer that, Whatever hinders a cause, of its very nature hinders the effect likewise. Now consent is the cause of matrimony, as stated above (Question 45, Article 1). Hence whatever voids the consent, voids marriage. Now consent is an act of the will, presupposing an act of the intellect; and if the first be lacking, the second must needs be lacking also. Hence, when error hinders knowledge, there follows a defect in the consent also, and consequently in the marriage. Therefore it is possible according to the natural law for error to void marriage.
Reply to Objection 1. Speaking simply, ignorance differs from error, because ignorance does not of its very nature imply an act of knowledge, while error supposes a wrong judgment of reason about something. However, as regards being an impediment to the voluntary, it differs not whether we call it ignorance or error, since no ignorance can be an impediment to the voluntary, unless it have error in conjunction with it, because the will's act presupposes an estimate or judgment about something which is the object of the will. Wherefore if there be ignorance there must needs be error; and for this reason error is set down as being the proximate cause.
Reply to Objection 2. Although error is not of itself contrary to matrimony, it is contrary thereto as regards the cause of marriage.
Reply to Objection 3. The character of baptism is not caused directly by the intention of the baptizer, but by the material element applied outwardly; and the intention is effective only as directing the material element to its effect; whereas the marriage tie is caused by the consent directly. Hence the comparison fails.
Reply to Objection 4. According to the Master (Sent. iv, D, 30) the marriage between Lia and Jacob was effected not by their coming together, which happened through an error, but by their consent, which followed afterwards. Yet both are clearly to be excused from sin (Sent. iv, D, 30).
Source: Summa Theologica (New Advent)