strange fire
Fire "from before the Lord" had kindled upon the altar of burnt-offering the fire which the care of the priests was to keep burning (Leviticus 6:12). No commandment had yet been given (Leviticus 16:12) how the incense should be kindled. The sin of Nadab and Abihu was in acting in the things of God without seeking the mind of God. It was "will worship" (Colossians 2:23) which often has a "show of wisdom and humility." It typifies any use of carnal means to kindle the fire of devotion and praise.