The high priesthood of Israel was inaugurated in Aaron and passed down from father to oldest son, unless that son died or was disqualified, as in the case of Aaron’s two oldest sons, who sinned against Jehovah and died. (Le 10:1, 2)

Additionally, fire holders were used to burn incense. (Le 10:1)

Aaron’s sons Nadab and Abihu used their fire holders to offer illegitimate fire before Jehovah and lost their lives as a result. (Le 10:1, 2)

When he and his two surviving sons were instructed not to mourn over the dead transgressors, “they did according to Moses’ word.”​—Le 10:1-11.

At this a fire came out from before Jehovah and consumed them, so that they died before Jehovah.” (Le 10:1, 2)

Any disrespect or violation of divine instructions in approaching the Sovereign God brought the death penalty, as in the case of two of Aaron’s own sons. (Le 10:1-3, 8-11; 16:1)

Ex 18:5–40:38; Le 1:1–27:34; Nu 1:1–10:32

Shortly after the dedication of the tabernacle and the consecration of Aaron and his sons for the priesthood (Le 8), Nadab and Abihu offered up illegitimate fire to Jehovah and were killed by fire proceeding from Jehovah. (Le 10:1, 2; Nu 3:2-4)

At Leviticus 10:1 the Hebrew word zar (feminine, za·rahʹ; literally, strange) is used with regard to “illegitimate fire, which [God] had not prescribed for them” but which Aaron’s sons Nadab and Abihu presented before Jehovah and for which he executed them by fire. (Le 10:2; Nu 3:4; 26:61)

Soon after the tabernacle service began to function, Aaron’s two sons Nadab and Abihu were struck dead by Jehovah for attempting to offer up incense with “illegitimate fire.” (Le 10:1, 2; compare Ex 30:9; see ABIHU.)

Later, Ithamar and Eleazar received a greater portion of priestly duties as Jehovah progressively outlined these. (Le 10:1-20)

They could approach him only in the way he approved (Nu 3:10; Le 10:1-3; 16:1)