The Muslim should avoid using this phrase, “I am going to think of you as if Allah never created you”, because the literal meaning is not sound or appropriate. It is not permissible for the Muslim to come to something that Allah (may He be exalted) has created and think of it as if it was never created, because it is not appropriate to think of something that exists as being non-existent. The Muslim is enjoined to guard his tongue and avoid ambiguous phrases that could be misinterpreted, especially phrases that refer to Allah (may He be exalted) or His Prophets or His religion. This phrase also constitutes tyranny and transgression, and boasting of one’s strength, which is not a praiseworthy action.

Summary of the answer: It is not permissible for a Muslim to accuse his fellow Muslim of being a Jew, a Christian or the like, even if he only meant that he has some of the (stereotypical) characteristics of the Jews, Christians or others.