Charles Spurgeon Commentary 1 Chronicles 21:26-27

"And David built there an altar unto Jehovah, and offered burnt-offerings and peace-offerings, and called upon Jehovah; and he answered him from heaven by fire upon the altar of burnt-offering. And Jehovah commanded the angel; and he put up his sword again into the sheath thereof." — 1 Chronicles 21:26-27 (ASV)

And David built there an altar unto the LORD, and offered burnt offerings and peace offerings, and called upon the LORD; and he answered him from heaven by fire upon the altar of burnt offering. And the LORD commanded the angel; and he put up his sword again into the sheath thereof.

That God had already done in his own intent and purpose, he now does it actually, just as it was before Jesus Christ, our great sacrifice, was offered. God, in the eternal purpose, had stayed the sword of vengeance from his redeemed people, and then actually did it when Christ their sacrifice was presented.