Is it permissible to attend lectures given by an atheist university professor, and what is the ruling on attending lessons given by non-Muslim professors who secretly invite students to church and urge them to convert to Christianity?

The fundamental principle is the permissibility for a Muslim to learn from an atheist or from followers of other disbelieving religions, just as the Companions learned writing from the prisoners of Badr. This is conditional upon the learning not leading to a Muslim being tested in their faith. If the teacher does not advocate for a violation of Islamic law and does not teach anything contrary to religion, then there is no objection to attending their classes. However, for teachers who call for abandoning Islam, it is not permissible to attend their classes; rather, the administration or competent authorities must be informed of this evil to have it changed.