He is the All-Glorious!
O you who are clinging fast to the hem of Covenant, your letter was read and your detailed questions were noted. Although calamities, like deadly poison, have affected my limbs, my members and my joints, such that my pen is prevented from writing and my tongue from speaking, and my tasks are so many that it is not possible to describe them, yet nevertheless, out of the great love that this servant has for that gentleman, a spiritual answer will be given, which will be compatible with Divine wisdom, concise and illuminating, perfectly explaining the matter. It will be a comprehensive explanation concerning this question and will contain acceptable and sought-after insights such that by this explanation, clarification, analysis, allusion, commentary and spiritual interpretation, one hundred doors will be opened up by each of its doors. Otherwise, were the horizons to become pages, it would not be enough to encompass [this theme].
You have asked about the wisdom of assigning some of the important legislation (ahkám) to the House of Justice. The first [point to be made] is this that this divine cycle is purely spiritual (rawhání), divine (rahmání) and moral (vujdání). It is not so much concerned with the physical (jismání), the worldly (mulkí) or with the stages of material existence (shu'ún-i násutí). Similarly, the Christian cycle was purely spiritual and in the entirety of the Gospels, there is nothing except the prohibition of divorce and an allusion to the lifting of the [law of the] Sabbath. All of the laws (ahkám) are spiritual and the morals divine. Just as it is said: "The Son of Man did not come to judge the world but to give it life." [cf. Jn 3:17; 12:47]. Now this great cycle is also purely spiritual and is the giver of eternal life, for the fundamental basis of the religion of God is to adorn [people] with good character, to improve them with virtuous conduct and to regulate their interactions. The intention is this that beings who were veiled [from the light] might attain to the vision [of His Beauty] and that darksome reality might become filled with light.
As for the other commandments, they are derivatives of certitude, faith, assurance and mystical insight. Nevertheless, because this blessed cycle is the most mighty of divine dispensations, it encompasses all of the spiritual and physical aspects [of human life] (marátib-i rawhaní va jismání) and is perfect in its power and sovereignty (quvvat va saltanat). Therefore those universal (all-encompassing) matters which are the foundations of the holy law (sharí`at) of God are revealed (mansús) [in the scriptures] and all secondary (subsidiary) matters (mutafarri`át) are to be referred to the House of Justice.
The wisdom of this is that time does not stand still. Change and alteration are among the specific and necessary conditions of contingent existence and of time and space. Therefore the House of Justice is able to act in accordance with the needs of the time (exigencies). It should not be thought that the House of Justice acts on the basis of its own thoughts and opinions. God forbid! The Universal (Most Mighty) House of Justice (bayt al-`adl-i a`zam) will make its decisions and enact its laws through the inspiration of the Holy Spirit (rúh al-quds), for it is under the guard, protection, and succour of the Ancient Beauty. Whatsoever it decides is obligatory, indisputable, necessary, and definitive for all. There is no recourse for anyone.
Say: O people! The Universal House of Justice is [sheltered] beneath the wing of your Lord, the Merciful, the Compassionate - that is to say, under His protection, His defence, His safe-keeping, and His guard. For He has ordered the believers and the assured ones to obey this goodly and pure group, this holy and victorious assemblage. Therefore its sovereignty is heavenly and divine and its laws (ahkám, ordinances) are inspired and spiritual.
Thus, this is the intention and the wisdom of referring social ordinances (ahkám-i madaniyyih) to the Universal House of Justice. In the holy law (sharí`at) of Islam (Furqán), also, all of the laws were not revealed (mansús) [in the Qur'an]. Indeed, not one thousandth were revealed. Although all important matters were mentioned, yet one hundred thousand laws were not mentioned. Later the `ulama derived (istinbát) them according to the rules (qavá'id) of the [science of] the Principles [of Jurisprudence] (úsúl). In those early [schools] of law (shará'í`), the individual members of the `ulama would derive (istinbát) these [laws] differently and they were implemented. Now, the [process of] deriving [the law] is to be referred to the House of Justice and the derivation (istinbát) and elicitation (istikhráj) of individual learned persons (`ulamá) has no authority, unless the House of Justice endorses it. The difference is just this that from the derivations and endorsements of the House of Justice, whose members are elected and have the confidence of the generality of the community, no differences will arise, whereas from the derivations (istinbát) of the members of the learned and wise comes about differences and this leads to sectarian splitting, separation and division. The unity of discourse and the oneness of the religion of God would disappear and the foundations of the law (sharí`at) of God would be shaken.
As for the command to marry, this is entirely a social law (ahkám-i madanniyat). Despite this, its conditions are stipulated and its fundamentals are made clear in the law (sharí`at) of God. However, the marriage of near relatives is not revealed [in the scripture] (ghayr-i mansús). It is referred to the House of Justice, who will make decisions based on the principles of civilisation (qavá'id-i madaniyyat), the exigencies of medicine, wisdom, and the tendencies of human nature. There is no doubt that (marriage with) distant stock is closer to the principles of civilization, medicine, and nature, than with closely related peoples. And consider this observation: in Christian holy law (sharí`at), although marriage to near relatives (aqárib) was in fact permitted, in that its prohibition was not revealed [in scripture] (mansús), nevertheless, the early Christian councils prohibited the marriage of near relatives to seven degrees of separation (literally "seven generations" - i.e. those who have a common ancestor seven generations back). Moreover this is implemented in all of the sects of Christianity because this is purely a social (madaní) matter. Now anything the House of Justice decides in this matter, that is the definitive and decisive divine law. No-one may infringe it.
If you consider it, you will see how much this referral of social laws (ahkám-i madaniyyih) to the House of Justice is consistent with wisdom. For whenever a difficulty arises because a compelling circumstance has arisen, at that time, because the House of Justice has decided the previous ruling (qarár), a particular House of Justice can again, because of specific compelling circumstances, issue a new specific ruling for this particular case and circumstance, and thus the danger may be completely averted. For whatsoever the House of Justice has decreed, that it can also abrogate.
In Islam, there was also the matter of punishments that were not decreed in the holy law (ta`zír). These were referred to those in authority. Since there was no revealed law (nasúsí) about the extent of such punishments (ta`zír), it was determined by and dependant upon the whim (ra'y) of the person in power. And these punishments ranged from verbal censure to the death penalty. This is what government (siyásat) of the people of Islam mostly revolved around.
In any case, the foundation of this mighty dispensation has been arranged in such a manner that its laws will be in accordance with and suitable for every age and time, unlike the holy laws (shará'í`) of the past, the implementation of which are now suspended or impossible. For example, observe that the laws of the Torah are in no way capable of being implemented today, since there are ten capital offenses in it. Similarly, according to the holy law (sharí`at) of Islam (Furqán), the hand is to be cut off for stealing ten dirhams. Now, is the implementation of such a law possible? No! By God! But this holy and divine law is compatible with all ages and centuries and the passing of time. "Thus have we made you a middle people, that you may be a witness unto the people and the Messenger may be a witness to you." (Qur'an 2:137).
The eloquent poetry and the consummate verses that have been composed with delectable contents should be recited and sung. Truly they are worthy of being chanted in the assemblies of divine unity.
Upon you be glory.
'Abdu'l-Bahá `Abbás
(source: https://www.momen.org/relstud/index.htm)