Is it permissible to learn knowledge from someone who is hostile to Islam and tries to turn new Muslims away from their religion, even if they are expert and competent in their field and explanation?

There is no harm in acquiring beneficial and permissible knowledge from a non-Muslim teacher, as long as the harm is safeguarded against. However, if there is certain or probable harm, such as the teacher being someone who raises doubts about Islam, then it is not permissible to learn from them, except for a student of knowledge who is well-versed in the Islamic sciences and is secure from being affected by those doubts.