What is the ruling on listening to missionaries in European countries out of curiosity to learn what they say about their religion?

The fundamental principle is that it is impermissible to listen to the people of falsehood, due to the implications of acknowledging them, increasing their numbers, and the temptation of doubts for the heart. Warning against non-Muslims is more important than warning against the people of innovation. However, it is permissible to listen to them if a legitimate benefit results from it, such as refuting their falsehood by someone firmly grounded in knowledge.