Was it correct to remain in the mosque and glorify God during the polytheistic sermon, or to leave, with the consequent stepping over people's necks and missing the Friday prayer?

The one who utters polytheism is not judged to be an unbeliever merely by uttering it, due to the existence of impediments to declaring someone a disbeliever, such as ignorance and misinterpretation. Therefore, prayer behind him is valid. The preacher should be advised, and if he does not desist, his matter must be reported to the responsible authority. There is no harm in silently glorifying Allah (Tasbeeh) when he speaks polytheism, for one is not obliged to listen to objectionable speech. Some jurists have permitted light remembrance of Allah (dhikr) if there is a reason for it.