Charles Ellicott 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 (the) covenant. —Jeremiah 34:10. The phrase means that they bound themselves by an oath (2 Chronicles 15:14). Compare to Nehemiah 10:30.

To seek the Lord ... with all their heart ... — See the same phrase in Deuteronomy 4:29.