The magistrate calls meursault “monsieur antichrist” because meursault believes that he is god meursault does not believe in god meursault believes that he is not religious enough meursault will not admit that he is guilty of murder

The magistrate calls Meursault “Monsieur Antichrist” because: Meursault does not believe in God
The Stranger's theme is more of Nihilism. It is a philosophical belief that "nothing matters" or has significance. It denies the existence of God or of meaning in life.

‘the resulting drop in pressure would be detected by the soviets.’