Shaykh al-Ṣadūq acknowledges that the large number and growth of the Imāmiyya were due to the Abbasid state

Shaykh al-Ṣadūq (d. 381 AH) acknowledges that the abundance and increase of the Imāmiyya were thanks to the Abbasid state and its officials. This, according to the author, supports the claim that this line of thought was an Abbasid construct, adopted to divide people and weaken them from supporting the leaders among the descendants of al-Ḥasan and al-Ḥusayn — the revolutionary Imams who rose against Abbasid injustice — and to occupy them instead with ideas of awaiting (the Imam), the doctrine of occultation, and similar notions.

Shaykh al-Ṣadūq said:

“Response to an objection: An ignorant objector, unaware of the traces of wisdom and heedless of sound governance for the people of the faith, may object by saying: Why did the occultation occur with the Master of your Age rather than with his preceding forefathers — the Imams, according to your claim — when we find that the Shīʿa of the family of Muḥammad (peace be upon them) in our time are in a better condition and more comfortable livelihood than they were during the Umayyad period, when they were forced to renounce the Commander of the Faithful (peace be upon him) and suffered killing and displacement? Yet now they live in safety and peace: their followers have become numerous, their supporters abundant, and their word has become manifest through the support of the كبار (leading figures) of the state and those among them who possess authority and power.”
— Kamāl al-Dīn, 1/45

The cited evidence, according to the author, is his statement about the Abbasids and their men supporting their doctrine:

“They are now at ease and secure; their followers have multiplied, their supporters increased, and their position has become manifest through the patronage of the كبار of the state and people of authority and strength among them.”

Furthermore, the author says that from the words of Ibn Abī Ṭayy, a historian of the Imāmiyya, it is known how Shaykh al-Mufīd (d. 413 AH) held a notable position with the Abbasid caliphs. As for Shaykh al-Ṭūsī (d. 460 AH), his situation is even more famous. Al-Khāqānī — one of the Imāmī scholars — said:

“The caliph al-Qāʾim bi-Amr Allāh al-ʿAbbāsī appointed him to the chair of theological discourse, through which he addressed both elites and commoners, even on the issue of the Imamate.”
— Rijāl al-Khāqānī, 2/102

And Sayyid Muḥsin al-Amīn mentions:

“Al-Ṭūsī attained such attention and esteem that the reigning caliph al-Qāʾim bi-Amr Allāh ʿAbd Allāh ibn al-Qādir bi-Allāh Aḥmad granted him the chair of theology and instruction — a chair that at that time possessed immense prestige beyond description.”
— Aʿyān al-Shīʿa, 9/159

The author concludes that whoever studies the intellectual history of the Imāmiyya regarding the promotion of the idea that a son was born to Imam al-Ḥasan al-ʿAskarī among the Shīʿa will find that its origin, in his view, lay in the Abbasid court: first to establish the notion of existence through investigation, then to develop the doctrine of occultation out of fear; meanwhile ignorance played a role, along with those who benefited financially, and those who fabricated claims to preserve the prior doctrinal idea that the Imamate continues through lineage — as well as sectarian loyalty that overlooked devotion to God. Because some people, after the passage of time, might assume that the growth of the Imāmiyya came from the strength of their proof.

May God grant you success.
O God, send blessings and peace upon Muḥammad and the family of Muḥammad.

Translation of Ustadh Kadhim Al Zaydi (May Allah reward him)