The secret taking of an object against the legitimate owner's reasonable will for the purpose of gain. If secrecy is absent, the act is called robbery. If the lawful owner is not reasonably opposed to the act, no theft is committed. And if the purpose of gain is absent, the taking of an object is rather a matter of damage. Consequently it is not theft if the owner consents, expressly or tacitly, or if an object is taken for reasons of extreme necessity, or as occult compensation. Thus, if a wife takes from her husband, either absent or unreasonably opposed as an avaricious man, something necessary for herself, for the support and benefit of her children, for reasons of charity in keeping with the family's financial condition or for helping parents in grave need, it is not theft.
Generally speaking, theft is a serious sin. According to St. Paul, "Thieves, usurers . . . and swindlers will never inherit the kingdom of God" (I Corinthians 6:10). But as an owner may be opposed in different ways to the loss of property belonging to him or her, so too the sin of theft admits of degrees, even to the point of constituting only a slight sin. Moreover, opposition to a loss may be based on the value or quantity of the stolen goods. Finally, theft is more or less grave according to the manner in which it is committed. A person may be more opposed to a large theft committed at one time than to a series of small thefts although amounting to the same value. Yet repeated petty thefts – venial sins if taken separately – may become a mortal sin either because of the intention or because of the conspiracy with which they are perpetrated or because the frequency of small thefts really constitutes a single large act of thievery.