Which is more harmful to religion: viewing lewd pictures of women or mocking religion? And does the ruling differ if the pictures are of non-Muslim women and the mockers are those who claim to be Muslim?

Looking at pictures of naked women is haram (forbidden) and a sin, and the one who commits it has violated Allah's command to lower the gaze. As for ridiculing religion, it is kufr (disbelief) that expels one from the fold of Islam, whether done in jest or seriously, because Allah Almighty says: "Make no excuse; you have disbelieved after your belief." [At-Tawbah: 66]. Ridiculing religion is one of the greatest diseases of shubuhat (doubts) and corrupts the heart. Both matters are grave and are a source of evil and fitna (temptation/strife) in religion, and one must beware of them.