And He (the Exalted) said: {And leave the outward of sin and its inward. Indeed, those who earn sin will be recompensed for what they used to commit.} (Al-An'am 120).

As for Reasoning, I say: If it were permissible to believe [exclusively] in all the outward forms, then it would be permissible for us to believe in the outward of all foods, drinks, and clothes and marriages, and utensils; and [it would lead] to praising the Three Opposites and their denying tribes; and to preferring Muhammad (PBUH) over all the Successors and the Nominal Stations; and to affirming the human state [Bashariyya] and its conditions upon them and upon all the Worlds of Light together. It would also lead to affirming the obligation of teaching the religion to women, and affirming marriage, eating, and drinking for the People of Paradise—while they neither marry nor are given in marriage, nor do they eat or drink. It would lead to affirming blame and excommunication [Takfir] upon whoever follows and believes in the Inward Religion of Monotheism, which is the religion of Allah and His core secret. This is because the People of the Outward profess a rejection of it, excommunicate those who follow it, and permit the seizing of their wealth, blood, and honor, while also rejecting belief in all the Celestial Lights and many similar things.