leviticus 10:1 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. - Free Bible Online

      Le 10:1-20. NADAB AND ABIHU BURNT.

      1. the sons of Aaron, &c.--If this incident occurred at the solemn period of the consecrating and dedicating the altar, these young men assumed an office which had been committed to Moses; or if it were some time after, it was an encroachment on duties which devolved on their father alone as the high priest. But the offense was of a far more aggravated nature than such a mere informality would imply. It consisted not only in their venturing unauthorized to perform the incense service--the highest and most solemn of the priestly offices--not only in their engaging together in a work which was the duty only of one, but in their presuming to intrude into the holy of holies, to which access was denied to all but the high priest alone. In this respect, "they offered strange fire before the Lord"; they were guilty of a presumptuous and unwarranted intrusion into a sacred office which did not belong to them. But their offense was more aggravated still; for instead of taking the fire which was put into their censers from the brazen altar, they seem to have been content with common fire and thus perpetrated an act which, considering the descent of the miraculous fire they had so recently witnessed and the solemn obligation under which they were laid to make use of that which was specially appropriated to the service of the altars, they betrayed a carelessness, an irreverence, a want of faith, most surprising and lamentable. A precedent of such evil tendency was dangerous, and it was imperatively necessary, therefore, as well for the priests themselves as for the sacred things, that a marked expression of the divine displeasure should be given for doing that which "God commanded them not."

JFB.