I mean, I quite often hear people talk about Greek mythology and Egyptian mythology referring to the religious beliefs and practices of ancient Greeks and Egyptians, perfectly equivalent to what Christians and Muslims in the modern era believe and practice, yet apparently, what Christians and Muslims believe and practice is not mythology…it’s something else. What is it then if not mythology?

there are still people who worship the greek gods, I am not the only one. If someone posts about the greek gods, even if they are talking about their religious beliefs, the first category suggestion will be mythology and folk tell. there are Hellenists, Greek neo pagans, Eclectic Greek Polytheist, ETC. it isn't dead beliefs.