Leviticus 10:20

When Moses heard that, he was content - The argument used by Aaron had in it both good sense and strong reason, and Moses, as a reasonable man, felt its force; and as God evidenced no kind of displeasure at this irregularity, which was, in a measure at least, justified by the present necessity, he thought proper to urge the matter no farther. Though the punishment of Nadab and Abihu may appear severe, because the sacred text does not specify clearly the nature and extent of their crime, we may rest assured that it was of such a nature as not only to justify but to demand such a punishment. God has here given us a full proof that he will not suffer human institutions to take the place of his own prescribed worship. It is true this is frequently done, for by many what is called natural religion is put in the place of Divine revelation; and God seems not to regard it: but though vengeance is not speedily executed on an evil work, and therefore the hearts of the children of men are set to do wickedness, yet God ceases not to be just; and those who have taken from or added to his words, or put their own inventions in their place, shall be reproved and found liars in the great day. His long-suffering leads to repentance; but if men will harden their hearts, and put their own ceremonies, rites, and creeds, in the place of Divine ordinances and eternal truths, they must expect to give an awful account to him who is shortly to judge the quick and the dead. Were the religion of Christ stripped of all that state policy, fleshly interest, and gross superstition have added to it, how plain and simple, and may we not add, how amiable and glorious, would it appear! Well may we say of human inventions in Divine worship what one said of the paintings on old cathedral windows, Their principal tendency is to prevent the light from coming in. Nadab and Abihu would perform the worship of God not according to his command, but in their own way; and God not only would not receive the sacrifice from their hands, but, while encompassing themselves with their own sparks, and warming themselves with their own fire, this had they from the hand of the Lord - they lay down in sorrow, for there went out a fire from the Lord, and devoured them. What is written above is to be understood of persons who make a religion for themselves, leaving Divine revelation; for, being wilfully ignorant of God's righteousness, they go about to establish their own. This is a high offense in the sight of God. Reader, God is a Spirit, and they who worship him must worship him in spirit and truth. Such worshippers the Father seeketh.

10:20 He rested satisfied with his answer. it appeared, that Aaron sincerely aimed at pleasing God: and those who do so, will find he is not extreme to mark what is done amiss.