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 more than a venial sin. An oath, therefore, taken to confirm detraction in a grave matter is mortally sinful; an oath confirming a boast about grave sins committed in the past is probably only venial.

Somewhat similarly in a promissory oath, if the act promised be gravely sinful, the defect of justice makes the oath gravely sinful; for it is a great insult to God to use his testimony to further what is mortally sinful. If the act promised is only a venial sin, a probable opinion^ holds that the oath is only venially sinful, against a more probable contrary view.

4. The effect of a promissory oath is to bind the person swearing to do what he promises by an additional obligation derived from the virtue of religion, which requires that out of reverence for God we should religiously perform what we promised under oath. If we fail to do this in a matter of moment, grave sin will be committed, as all acknowledge. Moreover, as is obvious, if when the oath is taken there is no intention of keeping it, a grave sin of perjury is committed, for perjury is never venial on account of triviality of matter. If, however, when the oath was taken there was a serious intention of keeping it, but afterwards there was a failure to do so in a matter of small moment, a probable opinion holds that such a want of fidelity in a small matter cannot be more than a venial sin.

5. The obligation imposed by a promissory oath is of strict interpretation, and follows the nature of the act or contract to which it is annexed, so that it is dependent on all the conditions and limitations with which law, or custom, or the circumstances invest the act or contract. For a promissory oath is accessory and follows the nature of the principal act to which it is annexed. And so an oath to observe the rules or statutes of a corporation is understood to refer only to those that are in force.

6. An oath should conduce to the service and honour of God; it cannot be a bond of iniquity; and so an oath to do what is wrong is sinful and of no effect. Similarly, an oath to do what is useless, or which hinders greater good, is null and void. In case, however, another party has acquired rights under an oath, justice requires that this should be kept, and so the mission oath, by which a sworn promise is given to serve a particular church or mission, binds a missionary priest even though he is persuaded that he has a vocation to the religious state.