The Kitáb-i-Aqdas - The Most Holy Book
Index term: Christ (Jesus)
Beware that ye take not unto yourselves more wives than two. Whoso contenteth himself with a single partner from among the maidservants of God, both he and she shall live in tranquillity.
While the text of the Kitáb-i-Aqdas appears to permit bigamy, Bahá'u'lláh counsels that tranquillity and contentment derive from monogamy. In another Tablet, He underlines the importance of the individual's acting in such a way as to "bring comfort to himself and to his partner". 'Abdu'l-Bahá, the authorized Interpreter of the Bahá'í Writings, states that in the text of the Aqdas monogamy is in effect enjoined. He elaborates this theme in a number of Tablets, including the following:

Know thou that polygamy is not permitted under the law of God, for contentment with one wife hath been clearly stipulated. Taking a second wife is made dependent upon equity and justice being upheld between the two wives, under all conditions. However, observance of justice and equity towards two wives is utterly impossible. The fact that bigamy has been made dependent upon an impossible condition is clear proof of its absolute prohibition. Therefore it is not permissible for a man to have more than one wife.

Polygamy is a very ancient practice among the majority of humanity. The introduction of monogamy has been only gradually accomplished by the Manifestations of God. Jesus, for example, did not prohibit polygamy, but abolished divorce except in the case of fornication; Muhammad limited the number of wives to four, but making plurality of wives contingent on justice, and reintroducing permission for divorce; Bahá'u'lláh, Who was revealing His Teachings in the milieu of a Muslim society, introduced the question of monogamy gradually in accordance with the principles of wisdom and the progressive unfoldment of His purpose. The fact that He left His followers with an infallible Interpreter of His Writings enabled Him to outwardly permit two wives in the Kitáb-i-Aqdas but uphold a condition that enabled 'Abdu'l-Bahá to elucidate later that the intention of the law was to enforce monogamy.
the "mystery of the Great Reversal in the Sign of the Sovereign"
Shaykh Ahmad-i-Ahsá'í (1753-1831), who was the founder of the Shaykhí School and the first of the "twin luminaries that heralded the advent of the Faith of the Báb", prophesied that at the appearance of the Promised One all things would be reversed, the last would be first, the first last. Bahá'u'lláh in one of His Tablets refers to the "symbol and allusion" of the "mystery of the Great Reversal in the Sign of the Sovereign". He states: "Through this reversal He hath caused the exalted to be abased and the abased to be exalted", and He recalls that "in the days of Jesus, it was those who were distinguished for their learning, the men of letters and religion, who denied Him, whilst humble fishermen made haste to gain admittance into the Kingdom" (see also note 172). For additional information about Shaykh Ahmad-i-Ahsá'í see The Dawn-Breakers, chapters 1 and 10.