What was the main religion in Samaria?

Answer 1

Everett Ferguson (Backgrounds of Early Christianity) says the earliest certain reference to a rival religious community in Samaria, based on Judaism are from the second century BCE, although Samaritanism evidently emerged in the post-Exilic period out of the common matrix that produced various other sectarian movements in Judaism.
Prior to this period, the Samarians were polytheistic, like the Israelites who had previously occupied the area.

Answer 2

Another answer from our community:

They were Jews, but separate from the Jews centered on the temple in Jerusalem.