Question: What is the ruling regarding comparative religion studies?
Answer:
It is good that we have amongst Muslims those who study and understand that (comparative religion). However, after knowing it, their preaching should be directed to the people of that (other) religion. It isn’t that you would now be telling Muslims in the mosque.
(Say for example): “Look up the book of Matthew Chapter 9 (and John Chapter 4. The two are conveying the same message. Aren’t they contradictory?”
Maybe if you show them of the errors in their texts, they may start having second thoughts about the validity of their creed.
You may see something like a commandment that they should observe salaat. But it’s song they sing and dance. You would now say “open so and so place (in your book). What were you asked to do? What do you do?”
We (Muslims) don’t believe it that that book they hold that Allah actually revealed. We hold that the one with them is first of all merely a translation that had been revised and modified overtime.
But when you go on to study it (comparative religion), it is to make them know that they are astray. There is no evil in that. Where evil lies is to take the Bible to the mosque and start saying “those people are misguided. This and that is what they say or do.” There is nothing we (Muslims) want to do with that.
During the lifetime of the Prophet ﷺ, he found Umar (RA) with a leaf of the Tawrah. The prophet asked him, “What is this?” Umar responded that “This is a leaflet from the Tawrah” The prophet’s eye suddenly changed colour and he said, “Do you want to replace the status of Qur’an? By Allah, if Musa, the son of Imran were to be alive, he would have followed me.” Umar immediately stopped reading it.
So, why would anyone now bring the Bible to the mosque? There is nothing we want to do with that. It is even more appalling that they now call some “Muslim scholar in the Tawrah”.
One Muslim cleric who is so fond of the Tawrah that he had read it this morning. They would go to the extent of saying: “The message in the Tawrah isn’t in the Qur’an.” What an unfortunate statement! He would go on reading it to their (the congregation’s) hearing portion to portion. Audhubillah!
If you find anyone doing so, caution them (and tell them) “It is the Qur’an you would read to our hearing. It is the sunnah of the Prophet ﷺ we would listen to. That is that about that.
Answered by Dr Sharafudeen Gbadebo Raji in the lecture titled “Who is a Scholar?” (2:08:30 – 2:11:16)
Translated from Yoruba by Abdussalam.
English 
Alhamdulilahi that he did not go against it.