2 Chronicles 26:18
And they withstood Uzziah the king, and said unto him, It appertaineth not unto thee, Uzziah, to burn incense unto the LORD, but to the priests the sons of Aaron, that are consecrated to burn incense: go out of the sanctuary; for thou hast trespassed; neither shall it be for thine honour from the LORD God.
Only the descendants of Aaron consecrated as priests were authorized to burn incense in the sanctuary, upon the altar. Uzziah trespassed by entering the sanctuary and temple to burn incense, an act of presumption against a plain command of God in stepping beyond the work God had appointed him. His royal position gave him no authority to perform the priests' sacred duties.
Azariah the high priest and his associates withstood the king with united protest, rightly opposing him and urging him to turn back. They ordered him to go out and leave the sanctuary, warning him that he had trespassed and that his act would bring him no honor from God. Made proud by his success, Uzziah answered with wrath and anger that he as king should be rebuked, standing in wrathful rebellion against the priests.
While still standing there he was suddenly smitten by divine judgment, with leprosy appearing on his forehead. In dismay he fled and was driven from the temple and never again entered the temple courts. As a leprous king he had to give up the scepter and live apart from society, remaining a leper until his death some years later.
His case remains a living example that departing from a plain declaration of the Lord brings the judgment of Heaven. Neither exalted position as king nor a long life of service excused his presumptuous sin, for God is no respecter of persons. That sin marred the closing years of his reign.