Under the figure of "King of Tyrus," Ezekiel declares that this great created being "had the seal of perfection, full of wisdom, and perfect in beauty." Ezekiel 28:12–19 describes Satan’s original state as the anointed cherub who was not only a created being, but created perfect (vss. 12-13). He enjoyed the highest position and honor in the presence of God (28:14, 16). Further, Isaiah spoke of him as Lucifer, “star of the morning , son of the dawn” - Isaiah 14:12-15. He was on the holy mountain of God and every precious stone was his covering. He was set there as "the anointed covering cherub" by God and walked up and down in the midst of stones of fire. He was perhaps the appointed guardian of the holiness of God, probably over this original planet earth. The inspired record says, "Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee" - Ezekiel. 28:12-15.
Scriptures relating to Lucifer/Satan's fall:
But what exactly was the character or nature of Satan’s iniquity? As suggested by 1 Timothy 3:6-7 and Ezekiel 28:17, though created perfect by God (Ezek. 28:13-15), it was conceit or pride that welled up in his heart because of his beauty and high position that led to arrogant aspirations and to his sin and fall. In Isaiah 14:12-15 and in Ezekiel 28:12-19, we have a remarkable account of the original place which Satan once had as "Lucifer, son of the morning." His fall from this original, exalted place, of perhaps the greatest of created angelic beings, is given in these Scriptures. These arrogant aspirations are described for us in Isaiah 14:12-17 : pride, selfwill, iniquity, rebellion, and violence are the reasons. Because of his pride and aspirations to be like God, he became God’s chief adversary (Heb. Satan). After this, he is never again called by any of these prestigious titles. Instead, he is called by terms that reflect his fallen character and hostility to God and men, like liar, murderer, Satan (adversary), the evil one, Abaddon (destruction), Apollyon (destroyer), Belial (worthless), serpent, and dragon.
Scotus, however points out that this sin was not pride properly so called, but should rather be described as a species of spiritual lust.
Although nothing definite can be known as to the precise nature of the probation of the angels and the manner in which many of them fell, many theologians have conjectured, with some show of probability, that the mystery of the Divine Incarnation was revealed to them, that they saw that a nature lower than their own was to be hypostatically united to the Person of God the Son, and that all the hierarchy of heaven must bow in adoration before the majesty of the Incarnate Word; and this, it is supposed, was the occasion of the pride of Lucifer.