The film's own opening credits are unusually candid about its method: it was written, in the writers' own words, in accordance with the Bible and also with Philo, Josephus, Eusebius, the Midrash and other ancient sources, and DeMille's screenwriters used those non-biblical historians specifically to fill in the decades of Moses's life at Pharaoh's court that Exodus itself passes over in a single verse. That gap is exactly where the film's most memorable invention sits: the decades-long rivalry between Moses and Rameses over the fictional princess Nefretiri, an entirely extrabiblical romantic and political plot with no equivalent in the Torah, built to give the years the text is silent about a dramatic shape.
Once the story reaches the events Exodus does narrate, the film follows the text far more closely: the plagues, the Passover instruction, the parting of the sea and the giving of the Law at Sinai all track their biblical originals in sequence, even where the special-effects staging is the film's own invention. The pattern is therefore not a film that departs from scripture so much as one that dramatizes scripture's own narrated events fairly closely while filling scripture's own silences with extrabiblical legend, exactly the material its credits name, and never claiming those filled sections come from Exodus itself.