Albert Barnes Commentary 2 Chronicles 23:6

"But let none come into the house of Jehovah, save the priests, and they that minister of the Levites; they shall come in, for they are holy: but all the people shall keep the charge of Jehovah." — 2 Chronicles 23:6 (ASV)

Keep the watch of the Lord — that is, guard against any attempt that might be made by the Baal-worshippers to force their way through the courts into the temple.