Is it permissible to travel to a land of disbelief for the purpose of calling people to Allah?

Question:
If a person emigrated from a land of disbelief to a land of Islam, is it permissible for him to return to the land of disbelief to call people to Allah?

Answer:
If he is qualified for da'wah, if he is qualified to call people to Allah, yes, he returns to call people to Allah. He returns to learn a trade or something beneficial. He returns for treatment. He returns for trade. There is no harm in traveling to the lands of the disbelievers for these purposes: to call people to Allah, for trade, and then he returns. As for learning the knowledge he needs, or that the Muslims need, from worldly matters, that is not religious knowledge. He does not go to learn that from the disbelievers; he learns it in the lands of the Muslims. But as for worldly sciences, he goes to learn them because, unfortunately, the disbelievers are more proficient in them than we are. He goes and learns them, and there is no harm in that.
And he is in that state, or for treatment: if he is afflicted with an illness and there is no treatment here, he travels because they are more capable of treatment than we are and they have the facilities. So he travels, but in all cases, he must hold fast to his religion. Yes, and he does not reside in the lands of the disbelievers except for the duration of the need, and then he returns. Yes.