My pet theory in high school was that Spinoza lived in a time when being an atheist could get you arrested and killed, so he came up with this Deus sive natura thing, which is a good cover and in practice is not too far from atheism.
I'm not sure it's explicitly a cover. I'm not sure he would have actually said "there is no God"; such a phrase might have been actually very difficult for someone to compose in that era, not just because it was shocking but it would be hard to think. Especially one raised in a devout Jewish community (from which he was expelled)
(I spent many years studying Spinoza, both in school and in private.)