Synopsis of the Books of the Bible, by John Nelson Darby, [1857-62], at sacred-texts.com
Lastly, we have what, alas! is always the case with man. The first day the priesthood is established, it comes short of the glory of God. Nadab and Abihu offer strange fire, acting as men in nature in their relationship with God, not founding their service on the altar of sacrifice, and they die.
The priests must on no account whatever depart from their consecration (Lev 10:6-7); therefore they must be Nazarites (Lev 10:9), apart from that which is only the excitement of the flesh, separated unto God from all that which would let nature loose in His presence; from that which would prevent them from feeling its power-a state of abstraction in which the flesh has no place. The presence of God must have its full power, and the flesh must be silent before Him. It is only thus that they would be able to discern between that which is unclean and that which is pure-that which is profane and that which is holy. There are lawful things, real joys, which, however, do not belong to priesthood-joys which flow from God's blessings, and which do not keep the flesh in check as does His presence; for there is always a certain restraint on the heart, on nature and its activity, produced by the presence of God. But priesthood is exercised before Him.