There is an incident in the biblical record that causes abiding consternation for many of God's people. It is the story of how two of the sons of Aaron, Nadab and Abihu, were slain suddenly by God.
Now Nadab and Abihu, the sons of Aaron, each took his censer and put fire in it and laid incense on it and offered unauthorized fire before the LORD, which he had not commanded them. And fire came out from before the LORD and consumed them, and they died before the LORD. Then Moses said to Aaron, "This is what the LORD has said: 'Among those who are near me I will be sanctified, and before all the people I will be glorified.'" And Aaron held his peace. (Lev. 10:1–3)
Aaron, of course, was the older brother of Moses and the first high priest of Israel. God had consecrated Aaron and his sons to the holy vocation of the priesthood. It was in the context of their priestly service that two of Aaron's four sons, Nadab and Abihu, each got a censer—a kind of vessel that was used in antiquity to contain the incense that was burned as an offering before God—put fire in them, put incense on them, and offered what the book of Leviticus calls "unauthorized fire."
What is "unauthorized fire," or, as it is rendered in other translations, "profane fire" or "strange fire"?