“And Nadab and Abihu, the sons of Aaron, took either of them his censer, and put fire therein, and put incense thereon, and offered strange fire before the Lord, which he commanded them not.
“And there went out fire from the Lord, and devoured them, and they died before the Lord.” – Leviticus 10:1, 2
In its letter the story of Nadab and Abihu seems to attribute vindictiveness to God. But spiritually it points to the truth that there are two fires of life possible to man, one of which leads to death and destruction and the other to life and salvation. Often in its letter the Bible seems to threaten. That is because the evil may in this way be restrained. To the good it serves as a warning, telling them to beware of the strange fire, that fire which will inflame the interior life with lusts so that they end up where their fire is not quenched.
The true fire on the altar is worship of the Lord from love in the heart. Any other fire is some foreign love in man which is not of heaven, and so must be from some other source.