Every conscious Shi‘a know that the Ahl al-Bayt’s (a.s.) knowledge originates from boundless Divine Knowledge, and the Almighty God has given them such knowledge that He has not given to anyone else except the Holy Prophet (S).
The purified Imams (a.s.) possess all the knowledge of the prophetic messengers and archangels. They are most learned about heavens and the earth and all in all they are the treasurers of Divine Sciences; there is nothing needed by people that is hidden from them.
However, the special conditions of any Imam and the instructions given to them by God according to the demands of the time, caused some of them to have more chances to expound religious issues and express their own widespread knowledge, such as Imam ‘Ali (a.s.), Imam al-Baqir (a.s.), Imam al-Sadiq (a.s.), and of course, Imam al-Ridha (a.s.), in whose era three important events unfolded in such a way as to cause him to overflow his knowledge into the Islamic society.
First: the Waqifiyya sedition and the misconceptions they raised concerning the Imamate of Imam al-Ridha (a.s.) prompting the Imam to guide people in this respect with his profound answers.
Second: destruction of the Barmakids in the fourth year of the Imam al-Ridha’s (a.s.) Imamate, leading to the elimination of one of the most important enemies of the Ahl al-Bayt (a.s.) in Harun’s regime at the hands of Harun, after which a relatively good chance was provided for the dissemination of the Ahl al-Bayt’s (a.s.) teachings.
Third: the issue of Imam al-Ridha’s (a.s.) heir apparency, which drew attention toward him, and the Abbasid Ma’mun who tried to humiliate his holiness before dignitaries of other religions and the scholars of religious schools by holding scientific gatherings and debate sessions; however, as the God Almighty manages the intrigue of the plotters and turns it against themselves, the very same debate sessions and circles led to familiarization with the scholarly eminence of Imam al-Ridha (a.s.) in such a way that great scholars of the time acknowledged his widespread knowledge and erudition.
Imam al-Ridha’s (a.s.) personality from the scholarly point of view is so sublime that Imam Ja‘far al-Sadiq (a.s.), the chief of Ja‘fari School, has told his son Musa b. Ja‘far about Imam al-Ridha (a.s.) before being born: “The learned one of the Prophet’s household (‘Alim Al-i Muhammad – S) is from your progeny; I wish I would meet him.”
Muhammad b. Musa Yaqtini said, “I have gathered fifteen thousand [topical] issues from among the answers that Imam al-Ridha (a.s.) gave to people.”
Aba Salt Hirawi, one of the notables of his time, said, “I have not seen anyone more learned than ‘Ali b. Musa al-Ridha; no scholar saw his holiness unless he would confirm this same witness of mine.
In numerous sessions, Ma’mun would gather a number of scholars of (various) religions, jurists of (Islamic) law, and theologians – who debated with the Imam (a.s.) – and the holy Imam defeated all of them in such a way that all of them acknowledged his moral excellence and their own helplessness.
And I heard him (Imam al-Ridha – a.s.) saying: ‘I would sit in the mosque (of the Prophet – S – in Medina) while there were many scholars there, whenever one of them was unable to answer a question, all of them would point at me and refer the problem to me to answer, and I would answer.”
Ibrahim b. ‘Abbas said, “I never heard anyone asking something from Imam al-Ridha (a.s.) and he would not know the answer. I saw no one more aware than him of the history of the past up to his own time. Ma’mun would constantly test him with various questions and he would give him full answers.”
Hasan b. ‘Ali al-Washsha’, who, due to the misconceptions raised by the opponents converted to the school of waqf (or Waqifiyya), said, “I wrote some questions in a scroll by which to test Imam al-Ridha (a.s.).
There was such a big crowd gathering at the door of the Imam’s house that I did not manage to meet him. All of a sudden I noticed that the Imam’s retainer is looking for me, asking: ‘Who is Hasan b. ‘Ali al-Washsha’, the grandson of Ilyas al-Baghdadi?’ I said, ‘That is me.’
He gave me a letter and said, ‘Here are the answers to the questions you have with you (and have not shown them to the Imam yet)!’ This way I became certain of his Imamate and gave up the school of waqf.”
Some of the sessions that Ma’mun had arranged were so magnificent and pompous and so many scholars of various nationalities from among the Jews, Christians, Sabaeans, Zoroastrians, and theologians had gathered that Hasan b. Muhammad Nawfili got terrified and advised the holy Imam and said, “They are people of fallacy and misconceptions; beware of them!”
Imam al-Ridha (a.s.) smiled and said, “Are you afraid of their defeating me? O Nawfili! Do you know when Ma’mun will repent this? When he hears my reasoning with the people of Torah with reference to their Torah, with the people of Bible with reference to their Bible, with the people of Psalms with reference to their Psalms, with Sabaeans in their Hebrew language, with Hirbads (Zoroastrians) in their Persian language, with the Romans in their Roman language, and with (other) eloquent orators in their own languages.
When I win them all and each group is defeated by my proofs and arguments and gives up their words and acknowledges the truth of my words, then Ma’mun will repent (holding a session), … La hawla wa la quwwata illa bi’Allah al-‘Ali al-‘Azim (There is no power and no strength save in Allah, the All-Exalted, the Almighty).”
 Tawhid of Saduq, see: Bihar al-Anwar, vol. 26, p.159.
 Basa’ir al-Darajat, see: Bihar al-Anwar, vol. 26, p.110.
 Ibid, p. 138.
 I‘lam al-Wara, 315.
 Al-Ghayba, Shaykh Tusi, p. 52, see: Bihar al-Anwar, vol. 49, p. 97.
 I‘lam al-Wara, 2/64; Bihar al-Anwar, 49/100.
 ‘Uyun Akhbar al-Ridha (a.s.), 2/180.
 A group of Shi‘as who denied the demise of Imam Musa b. Ja‘far, believing that he was the last Imam who was still alive. (Dihkhuda)
 Manaqib Al-i Abi Talib, vol. 3, p. 453.
 ‘Uyun Akhbar al-Ridha (a.s.), 2/139; al-Tawhid, 417.