John Gill Commentary 2 Chronicles 15:12

"And they entered into the covenant to seek Jehovah, the God of their fathers, with all their heart and with all their soul;" — 2 Chronicles 15:12 (ASV)

And they entered into a covenant
Asa and all his people; that is, as Piscator remarks, they went between the pieces of the calf cut asunder, for the confirmation of the covenant, see (Jeremiah 34:18) ,

to seek the Lord God of their fathers, with all their heart, and with
all their soul ;
to serve and worship him most truly, sincerely, and cordially.