You are to distinguish between the holy and the common, and between the unclean and the clean, and you are to teach the people of Israel all the statutes that the Lord has spoken to them by Moses.
The Lord spoke these words to Aaron after two of his sons, Nadab and Abihu, took a flippant attitude into their priestly duties and offered wrong incense to the Lord. God consumed them in a blaze of fire for their lack of regard for His holiness. And then he reminded their awestruck, grieving father of his own priestly obligations.
When we offend God's holiness, it is usually because we have only thought of ourselves. There are no details to describe exactly what the "strange fire" was that Aaron's sons offered. We just know that they tried to perform priestly duties in some way that God did not allow. It was the routine offering of incense and it wasn't done in a holy way. And they knew it. It appears to have been deliberate. They failed to distinguish what was holy in their actions by offering their incense in an unauthorized way.
Ministry seeks the glory of God first. It does not default to the perceived needs of the people first. It does not point to the ego of the minister first. God will be shown as holy in His worship and His service. His Word will be honored and taught. His holiness will mark His people. He will not stand for us giving Him anything less.