Introduction:
Nadab and Abihu were the eldest sons of Aaron, the brother of Moses and the first high priest of Israel. Their account is a significant account in the Old Testament, illustrating the importance of obedience to God's commands, particularly in matters of worship and priestly duties.

Biblical Account:
The primary account of Nadab and Abihu's sin is found in Leviticus 10:1-2. The Berean Standard Bible states: "Now Aaron’s sons Nadab and Abihu took their censers, put fire in them, and added incense; and they offered unauthorized fire before the LORD, contrary to His command. So fire came out from the presence of the LORD and consumed them, and they died before the LORD."

Context and Background:
Nadab and Abihu were part of the priestly lineage, having been consecrated alongside their father Aaron and their brothers Eleazar and Ithamar (Exodus 28:1). They had witnessed the divine presence and glory of God on Mount Sinai (Exodus 24:1, 9-11) and were given specific instructions regarding their priestly duties.

The Nature of Their Sin:
The sin of Nadab and Abihu is described as offering "unauthorized fire" or "strange fire" before the LORD. This act was a direct violation of God's explicit instructions regarding the sacrificial system and the use of incense (Exodus 30:9, Leviticus 16:12). The exact nature of their transgression is not detailed, but it is clear that they acted presumptuously and without divine sanction.

Theological Implications:
The incident underscores the holiness of God and the necessity for His people, especially those in leadership, to adhere strictly to His commands. The immediate and severe punishment of Nadab and Abihu serves as a sobering reminder of the consequences of disobedience and irreverence in worship. It highlights the principle that God is to be approached with reverence and in accordance with His revealed will.

Priestly Responsibility and Reverence:
Following the death of Nadab and Abihu, God spoke to Aaron, emphasizing the importance of distinguishing between the holy and the common, and between the clean and the unclean (Leviticus 10:10). This directive reinforced the need for priests to maintain a standard of holiness and to teach the Israelites God's statutes.

Lessons for Worship:
The account of Nadab and Abihu serves as a cautionary tale for all who approach God in worship. It emphasizes the importance of approaching God with the right heart and in accordance with His instructions. Worship is not to be taken lightly or performed according to personal preference but should align with the divine order established by God.

Conclusion:
The sin of Nadab and Abihu is a poignant reminder of the seriousness with which God views worship and obedience. Their account is a call to all believers to approach God with reverence, humility, and a commitment to His Word.