He didn’t make a mistake. He committed a sin. Sin is not a mistake. It is a choice of rejection of God. Sin is an offense against love and God is Love. It is not equivalent to someone slapping his neighbor but is equivalent to killing him. This is in light of the Passion of Jesus Christ. By sin, the creature kills the Lord, and so, the creature is dead. Jesus’ prophesy of His rejection is a stark understanding of the nature of sin and the one who commits a sin, though I am not sure He did intended His words to be read theologically so.
The angels were given a test of fidelity, similar to though not equivalent to the temptations we face, since man, unlike the angels, is meant to be married to God and so must have absolute alliegance to the Lord as a wife has for her husband. Satan disproved his fidelity. He committed a mortal sin: pride. Mortal sin requires knowledge, consent, and grave matter. The grave matter was pride, the consent was full, and the knowledge of God which the angels enjoyed was of such a superhuman degree that their choices were final. Because no man has superhuman knowledge of God, not even if God gave him special revelation, no man can say he is doomed to Hell for committing one mortal sin; he can still go to God for Mercy. But if he persists in his sin or dose not go to God, than he shall damn himself, just as the angels damned themselves. For God wills no one to Hell. He created all for Himself, and for a creature to be separated from Him is a great pain. But He chose to give us free-will, out of love, so that we might, out of love, choose Him. It is a matter of marriage, how a wife freely consents to be with her husband.
