15:3a And Abijam walked in all the sins that his father before him had committed,

Abijah sinned just like his father had done ⌊when he was king⌋.
-or-
Abijah sinned in the same ways that his father ⌊King Rehoboam⌋ had done/sinned.

15:3b and his heart was not as fully devoted to the LORD his God

His heart was not true to Yahweh his God,
-or-
He was not faithful/loyal to Yahweh his God

15:3c as the heart of David his forefather had been.

as the heart of his forefather David was true.
-or-
like his ancestor David.

15:3b-c (reordered)

His ancestor David had been fully devoted to Yahweh his God, but Abijah was not like that.

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