Concerning the exoteric and esoteric dimensions specifically, it is very natural to think of a religion as manifesting itself in its fullness and integrality at the inception of the revelation followed by a continual decline and occasional revivification in addition to the outward manifestation of various possibilities and adaptations prefigured in the revelation. This is pure speculation, however, so we must turn to qualified authorities for corroboration. Seyyed Hossein Nasr discussed this subject specifically in a section on 'The Sufi Tradition' in his book Three Muslim Sages.
"At the time of the Prophet, " he wrote, "when one might say the gates of heaven were open, the very intensity of the spiritual life and proximity to the source of the Revelation did not permit of a total separation of the Tradition into exoteric and esoteric, or Shari'ah and Tariqah, components, although both existed in essence from the beginning. The whole of the Tradition was at first like a molten lava in a state of fusion and did not 'congeal' and separate into its various elements until the withering influence of time and the corrupting conditions of this world had gradually 'cooled' and 'solidified' it. We see, therefore, that during the first two centuries of its life, while the Tradition was extremely strong, as proved by its rapid expansion and power of assimilation, there was neither a definitive codified school of law nor a clearly organized Sufi brotherhood, or order. We also note that in the 3rd/9th century the schools of law, or the Shari'ah became codified and at the same time Sufism began to manifest itself as a distinct element in the Islamic community, its doctrines and methods being propagated by brotherhoods, each of which was directed by a master and often named after him."
Although esoterism became primarily relegated to the Sufi brotherhoods within Sunni Islam, the union of exoterism and esoterism is more pronounced within Shi'ite Islam. It is true that Sufism was prevalent within Shi’ism up to the 11th/17th century when emphasis began to be placed upon Irfan (the tradition of Gnosis) the transmission of which seems to be more individual and less organized. However, even at a popular level Shi’ism partakes of a more pronounced esoteric character as evidenced by many of the prayers and litanies performed individually and as a group. The traditions or collected oral teachings of the Imans which complement those of the Sunnah contain many overtly esoteric teachings. Furthermore, the Iman as spiritual and temporal leader and representative of the Prophet (pbuh) possesses the same function within the Shi’ite community as does the Shaykh within an individual Sufi Brotherhood, that function of qutb or pir, literally “pole”.