John Gill Bible Commentary

And he slew it,.... Not Aaron, nor any of his sons, who as yet were not fully consecrated and installed into their office, but Moses, as follows: and Moses took the blood; which was received into a basin when the bullock was slain: and put it upon the horns of the altar round about with his finger; upon the four horns of the altar, which were at the four corners of it, and dipping his finger into the blood, he besmeared the horns with it, and drew it about with his finger here and there; and so is said to be done round about the altar, as these horns were: and purified the altar; or cleansed it; not from moral guilt and pollution, which it was incapable of, but from all ceremonial pollution it might be supposed to have: and poured the blood at the bottom of the altar; the rest of the blood he did not use about the horns: and sanctified it; separated it from common to sacred use: to make reconciliation upon it; that it might be fit to have sacrifices offered on it to make atonement and reconciliation for sins; for which reason it was necessary it should itself be pure and holy, in such sense it was capable of being so.