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The first angel to rebel against God was named Lucifer and he was a member of one of the nine choirs of angels called Cherubim. Lucifer started the angelic rebellion and continues to lead the demonic host hellbent on mankind's eternal damnation. All the angels were created good but became devils and man's sworn enemies, according to paragraph 393 of the Catechism of the Catholic Church, owing to the "irrevocable character of their choice."
The chief fallen angel is named in Isaiah 14:12, which reads, "How art thou fallen from heaven, O Lucifer?" In his Summa Theologica (P 1, Q 63, A 5, co.), St. Thomas Aquinas references this passage and explained that it "is said of the devil under the figure of the prince of Babylon." Saint Thomas further notes this same devil is the subject of Ezechiel 28:11–17, revealing that Lucifer belonged to the angelic choir of Cherubim.
If allowed by God, warns the Catechism in paragraph 395, fallen angels "may cause grave injuries — of a spiritual nature and, indirectly, even of a physical nature — to each man and to society." Chapters one and two of Job shows that God allows Satan to harm good men with plagues, bad weather and inspired attacks by evil men.
Our Lord freed many from the power of fallen angels by performing exorcisms. Mark 9:16–28 shows that without "prayer and fasting" even Christ's closest disciples weren't able to cast out certain devils. A bishop's permission is needed to formally conduct the Catholic Church's sacramental rite of Exorcism. The Church does offer to the laity, however, prayers they can recite to curb diabolic influence such as the Prayers Against the Powers of Darkness.
The various degrees of control that fallen angels exercise over their victims are called obsession, oppression and possession. Persons morally weakened by sin are more susceptible to demonic influence. Exorcists, therefore, wholeheartedly promote the sacrament of confession to help free souls from Lucifer and his minions.
Arm yourself against the influence of evil spirits with knowledge from season two of Church Militant's Premium show Armor of God—Temptation & Overcoming It.