… the purpose of the religion of God is the education of humanity and the unity and fellowship of mankind. Furthermore, we will establish the point that the foundations of the religions of God are one foundation. This foundation is not multiple, for it is reality itself. Reality does not admit of multiplicity, although each of the divine religions is separable into two divisions. One concerns the world of morality and the ethical training of human nature. It is directed to the advancement of the world of humanity in general; it reveals and inculcates the knowledge of God and makes possible the discovery of the verities of life. This is ideal and spiritual teaching, the essential quality of divine religion, and not subject to change or transformation. It is the one foundation of all the religions of God. Therefore, the religions are essentially one and the same.
The second classification or division comprises social laws and regulations applicable to human conduct. This is not the essential spiritual quality of religion. It is subject to change and transformation according to the exigencies and requirements of time and place. For instance, in the time of Noah certain requirements made it necessary that all seafood be allowable or lawful. During the time of the Abrahamic Prophethood it was considered allowable, because of a certain exigency, that a man should marry his aunt, even as Sarah was the sister of Abraham's mother. During the cycle of Adam it was lawful and expedient for a man to marry his own sister, even as Abel, Cain and Seth, the sons of Adam, married their sisters. But in the law of the Pentateuch revealed by Moses these marriages were forbidden and their custom and sanction abrogated. Other laws formerly valid were annulled during the time of Moses. For example, it was lawful in Abraham's cycle to eat the flesh of the camel, but during the time of Jacob this was prohibited. Such changes and transformations in the teaching of religion are applicable to the ordinary conditions of life, but they are not important or essential. Moses lived in the wilderness of Sinai where crime necessitated direct punishment. There were no penitentiaries or penalties of imprisonment. Therefore, according to the exigency of the time and place it was a law of God that an eye should be given for an eye and a tooth for a tooth. It would not be practicable to enforce this law at the present time -- for instance, to blind a man who accidentally blinded you. In the Torah there are many commands concerning the punishment of a murderer. It would not be allowable or possible to carry out these ordinances today. Human conditions and exigencies are such that even the question of capital punishment -- the one penalty which most nations have continued to enforce for murder -- is now under discussion by wise men who are debating its advisability. In fact, laws for the ordinary conditions of life are only valid temporarily. The exigencies of the time of Moses justified cutting off a man's hand for theft, but such a penalty is not allowable now. Time changes conditions, and laws change to suit conditions. We must remember that these changing laws are not the essentials; they are the accidentals of religion. The essential ordinances established by a Manifestation of God are spiritual; they concern moralities, the ethical development of man and faith in God. They are ideal and necessarily permanent -- expressions of the one foundation and not amenable to change or transformation. Therefore, the fundamental basis of the revealed religion of God is immutable, unchanging throughout the centuries, not subject to the varying conditions of the human world. (Abdu'l-Baha, The Promulgation of Universal Peace, pp. 365)