It is written concerning Satan, "How you have fallen from heaven, O star of the morning (that was his name - the shining one), O son of the dawn" (Isa.14:12)
As the head of the angels, Lucifer was in God's presence constantly. Why did he fall away? The reason is given in the next two verses (Isa.14:13,14) "Because you said in your heart 'I will ascend into heaven, I will exalt my throne above the stars of God....I will be like the Most High'.... Nevertheless you will be cast down into hell."
Lucifer was more gifted, more beautiful and had more supernatural abilities than all the other angels. He led the angels in the worship of God - until pride entered his heart. He then began to think, "Only One Person is above me, and that is God Himself. I will rule over Him as well."
That was really a stupid thought that came into his heart. How could he ever ascend above his Creator? But that is how Satan is - quite foolish in many of the things he does and thinks, even though he is so clever!! Many clever people in the world too do a lot of utterly foolish things, spiritually speaking. The more we study about Satan, the more we will see how stupid he is in many of the things he does. Satan was not happy to be where God had placed him. He wanted to ascend to the place where everyone would worship him! He did not say so. He just began to have such thoughts in his heart. We read that in verse 13. But God looks at our hearts, and he looked at Lucifer's heart, and saw what he was aiming at.
It did not take many years for the highest archangel to become the devil. No. Only a moment. He did not fall gradually, step-by-step. No. He came down like lightning, as Jesus said - in a moment (Lk.10:18). One moment, he was the shining one. As soon as he began nourishing the thought of becoming like God, immediately he became the devil.
How long does it take for an angel to become a devil? Not even one second. Just a moment. How long does it take for a really good person to become like the devil? Just a moment. That's all. Remember that.