‘ And Joshua said to all the people, “Thus says YHWH, the God of Israel, ‘Your fathers dwelt in olden days beyond the River, even Terah, the father of Abraham, and the father of Nahor, and they served other gods.' ” '
Joshua now began the preamble to the covenant, the declaring of the acts of YHWH on behalf of His people. He began with Terah the father of Abraham and his brother Nahor, pointing out that Terah and his family were worshippers of false gods. The River was the River Euphrates. Israel were ‘descended' from Terah through Abraham, and from Nahor through Rebekah.
This worshipping of false gods by Abraham's relatives is not mentioned elsewhere in the Old Testament, but it is a clear assumption from Abraham's call. He was called out from his family because of his new found faith in YHWH, probably garnered from reading the covenant records in the family archives which make up Genesis 1-11. There are other indications of it. Their connection was with the cities Ur and Haran, connected with Sin the moon god, (one of Terah's sons was named after Haran), the name Sarai (princess) may connect with Sharratu, the consort of Sin, some have suggested that Milcah may connect with Malkatu, a title of Ishtar (Inanna) (see Genesis 11:27-32). Rachel stole her father's ‘gods' (teraphim) - Genesis 31:19. Jacob's God was ‘the God of your father' (Genesis 31:29), and the God of Abraham is probably distinguished from the god of Nahor (Genesis 31:53).