Uzziah presumptuously disregarded God's Word, which says that only the priests were to do this responsibility. Success after success had spoiled his character. He became arrogant, conceited, filled with inordinate self-esteem. His heart was lifted up with pride, and he tried to emulate the Oriental kings around him and become a priest also.
John W. Ritenbaugh
Why Three Kings Are Missing From Matthew 1
What is God's reaction to presumptuous sin? There is no mercy! He immediately struck Uzziah with leprosy, and the man was cut off. It sounds like he was cut off from just about everything for the rest of his life. He pretty much lost his crown—lost everything. He certainly lost his health, all because he sinned presumptuously.
Ambition is not a trait that impresses God—certainly not presumptuous ambition. He is looking for a humble man who will stay where God has put him and do what God tells him to do. Uzziah was king over God's people Israel. Was that enough? No! To Uzziah, he wanted to be a priest as well.
It was his strength (verse 16), his pride, that drove him to do this presumptuous act. His heart was lifted up within him to make him think that he was worthy of more than what God had given him. He became discontent with his place. He was dissatisfied with what God had given him (his position as king); and he took to himself a position that was somebody else's.
God would never give him the authority to be a priest: He was a Jew, and the law says that only a son of Aaron could be a priest—a Levite. Uzziah knew that! But in spite of all the warnings, all of what God says, and in spite of what the priests themselves told him—he did it anyway.
Is it not interesting that he was struck with leprosy in his forehead? That should tell us something. What does leprosy stand for? What is it a symbol of? Defilement! It is a symbol of uncleanness—of being impure. Remember in the Pentateuch, all those rules about if somebody had a spot then they were to remain outside the camp? And they were to wash and do all various things. What they were looking for was leprosy. All the things that they had to do—all the washings, all the inspections, and everything else—were to certify whether the person was clean or unclean.
God put this uncleanness—this mark of defilement—right on Uzziah's forehead, where he could not hide it. What is right behind the forehead? The mind is the seat of intellect, as well as the seat of our character. The mind is where it is all being stored. That is where we think. God put this mark on this man's forehead to show that his character had been defiled—by presumptuousness, by this overweening pride that he was greater than what God had made him.
This is why presumptuousness is such a terrible, damnable sin—because it defiles character that has been built. It ruins it, to the point that God cannot work with it any more. He says that person shall be cut off from His people. There is no sacrifice for this kind of sin. That is how serious presumptuousness is.
Richard T. Ritenbaugh