Exodus 32:25-30

Again, this was right after the Golden Calf incident and Moses asked, "Who is on the Lord's side? Let him come to me." Is it not amazing that only one tribe jumped up? There is no mention of anyone else. Then all the sons of Levi came to Moses, and he sent them to do the Lord's bidding to uphold His law. And what did God do for the sons of Levi? He bestowed a blessing on them for they did not regard son or brother in upholding the truth of God. God's law was first priority, and God blessed them for it. Sin had entered the camp and it had to be eliminated. Unfortunately, this was not the only time that the Israelites would grieve the Holy Spirit.

Exodus 32:25-28

Now Aaron, who should have been the one who stood in the gap, and who should have been a wall so that calf was never built, fell short that time in Moses' absence, and so Moses called on anybody—volunteers—to stand in the gap and do the dirty work in the face of this sin. This is one of the Levites' finer moments. They rose to the occasion, and they did the dirty work. This is what I mean about standing sometimes in the gap, that it is hard, and requires sacrifice. I don't think it was easy for them to execute their fellow Israelites, but executioners they were, and it was dirty work.