What is the ruling on one who invokes mercy upon the leaders of disbelief and calls Muslims to do so, while knowing the prohibition of this matter and studying the fundamentals of religion, and what is the ruling on acquiring Islamic knowledge from him?

It is not permissible to supplicate for mercy upon disbelievers, due to Allah's saying: "Indeed, those who disbelieved and committed wrong – never will Allah forgive them, nor will He guide them to a path" [Quran 4:168]. This is because mercy is exclusive to the believers. Asking for forgiveness for polytheists is a transgression in supplication. However, there is no harm in benefiting from a non-Muslim or someone who errs in specific scientific matters, such as the ransom of the prisoners of Badr by their teaching the children of the Companions how to write. This is because forsaking benefit from someone whose methodology is flawed would lead to the loss of much knowledge. Nevertheless, one must be cautious of showing loyalty to disbelievers or atheism.