The Lord is a consuming fire (Deuteronomy 4:24; 9:3; Hebrews 12:29), and he tries our faith so that we might be found worthy to praise and honor him and glorify him before others in the things we do (1Peter 1:7). Nadab and Abihu, the sons of Aaron, were killed in the presence of the Lord when they offered strange fire before him, which the Lord had commanded them not to do (Leviticus 10:1-2). Whether or not they had been drinking when they had done this (cp. the Lord’s immediate command after their death in Leviticus 10:8-11) is not clearly stated, but they certainly did not discern from that which was common and that which was holy, something they should have known.
The text says Nadab and Abihu used strange fire in their censor to offer before the Lord (Leviticus 10:1), which was not supposed to be done. It was supposed to be taken from the altar whose fire was never to go out (Leviticus 6:12-13; 16:12-13; cp. Revelation 8:5). The Lord provided the fire on the altar (Leviticus 9:24), but they did not take their fire from there. They had obediently sacrificed their sin offering (Leviticus 10:19; 16:2), but for whatever reason did not utilize the fire provided by the Lord, and in so doing, the Lord could not be sanctified in them without claiming their lives (Leviticus 10:3; cp. Exodus 19:22). He, himself, discerned what was holy and what was common, and the people feared.
In Acts 5 Luke tells us of the offering of Ananias and Sapphira. Their offering is presented over against that of Barnabas in Acts 4:36-37. We discover in the text that they offered their offering under pretence (Acts 5:2). It seems they had an ulterior motive in doing what they did for the poor. It was not to satisfy the needy, nor to actively fulfill the word of God (Deuteronomy 15:1-5), but only to appear so. They didn’t have to offer anything (Acts 5:4a-b). It was in lying to God that they sinned (Acts 5:4c). Had they confessed that their offering represented only part of the sale, no wrong would have been committed, but as it occurred, they offered “strange” fire before the Lord. It was not from the fire of the cross (cp. Isaiah 10:16-17). Rather it was the “fire” of their own zeal. They were zealous to appear to be what they were not.
Luke also implies that the zeal/fire of Ananias and Sapphira was rooted in the zeal/desire of the enemy. Paul claims later that false brethren had crept in unawares to spy our out liberty (Galatians 2:4; cp. Jude 1:4). It would be ludicrous to believe that clandestine activity could be successful without enemies gaining the trust of leaders in the Church who had the authority to put them in positions of authority. Therefore, it seems probable that Luke intends for us to believe that Ananias and Sapphira were false brethren, and only the tip of the iceberg, for Acts 5:13 tells us that “None of the rest dared join them…” i.e. the Apostles. Who are “the rest” if not of the same sort as Ananias and Sapphira? It makes no sense to say “the rest” refers to other brethren, for why should any of the brethren be in fear be joined to the Apostles? Certainly Barnabas had no fear of this, nor did Paul later. The Seven mentioned in Acts 6 had no fear of being joined in authority with the Apostles. Why should anyone fear the Apostles, unless there was reason for “the rest” to conclude they would end up like Ananias and Sapphira, because they shared their zeal for another cause?
It would be naïve to believe that Annas and the Sadducees, who believed as he did, simply stood by and permitted the Apostles to do as they wished when the text clearly shows the Jewish authorities were unhappy with the spread of the Gospel (Acts 4:1-2, 6-7, 17-18, 21). Rather, having not found any lawful reason that could be used to keep the Gospel from spreading, it seems logical that the time between Acts 4 and 5 was the time when false brethren were planted among the believers in order to bring the Church into subjection to the Jewish leadership. Yet, the Lord, as a Consuming Fire, went before the disciples in the face of the giants (the Jewish authorities) who came against them (cp. Deuteronomy 9:2-3).