Kitáb-i-Aqdas

on the third offence, place ye a mark upon his brow so that, thus identified, he may not be accepted in the cities of God and His countries   ¶45

The mark to be placed on the thief's forehead serves the purpose of warning people of his proclivities. All details concerning the nature of the mark, how the mark is to be applied, how long it must be worn, on what conditions it may be removed, as well as the seriousness of various degrees of theft have been left by Bahá'u'lláh for the Universal House of Justice to determine when the law is applied.