When Imam Reza (P.B.U.H) came to Ma'mun, Ma'mun ordered Fazl-ibn Sahl to gather scholars like Catholicos, Ra's Al-Galot, Sabian elders, Zoroastrians, Romian Nostas and Rhetors so that he would listen to Imam Reza (P.B.U.H) and these scholars' remarks. Fazl-ibn Sahl did this and informed Ma'mun. Ma'mun said: "Bring them in". After greetings, Ma'mun told them: "I have brought you here for a good matter. I want you to debate with my cousin, who has come to me from Medina. Come here tomorrow early in the morning and don't be late."Hassan bin Mohammed Nawfali has said: “We were debating with Imam Reza (P.B.U.H) when Yasir entered. He dealt with Imam’s choirs. He told Imam: "My lord! Ma'mun extended his greetings and asked me to inform you that scholars and people from different religions and speakers of all denominations are present to him. If you wish to listen to them and think it's the right thing to do, please attend their meeting early in the morning. But if you don't wish that, please don't be hard on yourself. If you want them to get to your service it wouldn't be difficult for them". Abu Al-hassan said: "Send my reagrds to him and tell him that I realized what he had meant and by the will of God I'll come to see him early in the morning." Hassan ibn Nawfili says: "when Yaser left, he turned to me and said: " Nawfili! You are from Iraq and you aren’t harsh and rough. What is your opinion about the fact that your cousin has gathered the pagans for us?" I said: "My dearest! He wants to test you and know what you have. Imam said: "What are his actions based on?" I said: "People of Rhetorics and Heresy are unlike scholars, since a scholar doesn't reject anything except for evil, but speakers and orators and pagans are in denial. If you give them a reason that the Lord is unique they will say: "Prove his oneness." And if you say: "Mohammad is the messenger of Allah." They will say: "Prove his prophecy." Then they astound the person who is rejecting their reasons and make him mistaken so many times that he has to leave his words. Please be careful with them!" Imam smiled and said: "Nawfali! Are you afraid that they will stop me from reasoning?" I said: "No, I swear to God that I am not afraid and I hope dear Lord will help you overcome them" So Imam said: "Nawfali! Do you want to know, when Ma'mun will regret?" I said: "Yes." He said: "When I explain my reasons for the believers in Torah with their Torah, and for the ones believing in Bible with their Bible and for the believers in Psalms with their our book and for the Sabians being Hebrew and for Zoroastrians by being Persian and for the nation of Rome by being Roman and for the Rhetors by their own language. When I stop all of them from thinking and void all of their reasons, in a way that they stop speaking and accept my words, Ma'mun will realize that he doesn't deserve his position. Then he will regret and there is no power and strength unless by the will of the great God”.
Tomorrow morning Fazl Ibn-Sahl came and said: "My Lord! Your cousin is waiting for you and people have gathered too. What is right to do in your opinion?" Imam Reza (P.B.U.H) said: "you go first and I'll come after you, God willing." and then he performed the ablution to say prayers. And then left and we left with him until we arrived to Ma'mun’s place. There were many people there and Muhammad ibn-Jafar was among a group of Talebians and Hashemians and the heads of traibes were there too. As Imam (P.B.U.H) entered, Ma'mun stood up and Mohammed ibn-Jafar and all of Bani Hashem also rose. They were standing until Imam Reza (P.B.U.H) and Ma'mun sat down and he ordered them to sit. Ma'mun was talking to Imam (P.B.U.H) for about an hour. Then he said: "Catholicos! This is my cousin Ali ibn Musa ibn Jafar, the son of Fatimah, the daughter of our prophet and the son of Ali ibn Abi Talib (P.B.U.H). I want you to speak with him and debate with him and be fair."
Great tradition narrators like Sheykh Tabarsi (1403 AH, No 2: 416), Sheykh Saduq (1378 AH, No 1: 156) and Allameh Majlesi (1403 AH. No10: 301), have quoted Imam Reza's (P.B.U.H) full text debate and reported that Catholicos said to Ma'mun: "Oh MA'mun! How would I debate with someone that his reasons are from a book that I don't believe in and believes in the prophet that I don't?" Imam Reza (P.B.U.H) replied: "Nazarene! Will you accept, if I ratiocinate from Bible?" he then added: "ask whatever you want and then pay attention to the answers!" and the debate between the eighth of Imams and this major Christian figure (equivalent of Patriarch in modern times) started. In a part of this debate, Catholicos says: "anyone who can revive the dead, heal the blind and the leper, he is a lord and worthy of worship".
Imam Reza (P.B.U.H) exemplified some instances to correct this false belief. He said addressing to Catholicos: "Elisha did what Jesus did. He walked on water, revived the dead and healed the blind and the leper. But his people didn't place him as their Lord and no one worshiped him. Ezekiel also did what Jesus did. He revived 35000 people, 60 years after their deaths". Imam Reza (P.B.U.H) mentions another instance in this matter. Then continues: "if you know Jesus as your Lord, then Elisha and Ezekiel should be your Lord too, since they did exactly what Jesus did. About one thousand of Israelites escaped from their nation in fear of plague. But the Lord took all of their lives in one moment. Their bones turned into dust. After a while, a prophet from Bani Israel passed there. He was surprised of those many carious bones (and seeing that scene). Lord revealed to him: do you want me to revive them for you so that you can warn them? He said: yes, my Lord! God Revealed to him: call them! He said: Carious bones! Rise up by the will of God! All of them stood up as they were cleaning dust from their faces. Prophet Abraham [also did this], when he fragmented those birds into pieces and put each part of them on a mountain then called them and they [also] ran towards him immediately. Moses Ibin Omran also did this. He chose 70 of his followers and they went to the mountains. When they arrived there, told Moses: you have seen the glorious Lord. So show him to us as you have seen him! He said: I haven't seen him. They said: we'll never believe in you until we see the Lord openly. Then a thunderbolt speared over them and burned all of them. Moses was left alone and told God: My Lord! I chose 70 people from BaniIsrael and brought them here. Now I should get back alone! How will my nation acknowledge my news? If you wanted to perish them you should have perished them and me before. Have you perished us because of our foolish behavior? So God revived them after death. After expressing these instances, Imam stated, addressing his opponent in the debate: ‘You can't deny these instances; since Torah, Psalms and Furqan have talked about them. So if anyone who revives the dead and heals the blind and the leper is a Lord other than the great God, you should accept all of them as the Lord!’
Source: "Translation and Commentary of Imam Reza's (P.B.U.H) Arguments", by Imam Reza's (P.B.U.H) International Foundation for Culture and Arts Publications