Charles Ellicott Commentary 2 Chronicles 29:24

"and the priests killed them, and they made a sin-offering with their blood upon the altar, to make atonement for all Israel; for the king commanded [that] the burnt-offering and the sin-offering [should be made] for all Israel." — 2 Chronicles 29:24 (ASV)

Made reconciliation with their blood upon the altar. —Literally, made sin offering of their blood. (Compare to Leviticus 9:15.) The meaning may be seen by reference to Leviticus 4:30 and following. The priest dipped his finger in the blood of the victim and touched the horns of the altar with it, and then poured the blood at the base of the altar.

For the king commanded ... Israel. — For for all Israel the king had commanded the burnt offering and the sin offering; or, for “For all Israel,” said the king, “is the burnt offering and the sin offering.” The expression all Israel includes the northern kingdom. (Compare to Hezekiah’s invitation to its people to attend the Passover, 2 Chronicles 30:1.)