The Kitáb-i-Aqdas - The Most Holy Book
Index term: mark on the brow of thief, under punishments specified; Theft
k45.
Exile and imprisonment are decreed for the thief, and, 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. Beware lest, through compassion, ye neglect to carry out the statutes of the religion of God; do that which hath been bidden you by Him Who is compassionate and merciful. We school you with the rod of wisdom and laws, like unto the father who educateth his son, and this for naught but the protection of your own selves and the elevation of your stations. By My life, were ye to discover what We have desired for you in revealing Our holy laws, ye would offer up your very souls for this sacred, this mighty, and most exalted Faith.
n71.
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
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.