Paul seemed somewhat surprised by the Thessalonians’ lack of discernment over doctrines he had already taught them (vs. 5).
Moses gave these instructions to Aaron’s sons, Eleazar and Ithamar, on the occasion of the fiery death of their two older brothers, Nadab and Abihu.
As Aaron’s sons, Nadab and Abihu served as priests in the Tabernacle. As priests they were tasked with keeping a fire burning constantly on the bronze altar of the Tabernacle and supplying coals from the altar for burning incense in the Tabernacle.
Perhaps careless from drinking wine, Nadab and Abihu took coals from another source. This violation of God’s instructions resulted in God’s judgment on them and their subsequent deaths.