Q: “What was the sin by Satan which caused God to cast out this originally holy angel?”
A: No answer to this can be ascertained with irrefutable Scripture references. From some apparent inferences, however, we can suggest that it was likely pride or envy. Luther was of the opinion that it was pride. A theologian by the name of Quenstedt felt that the remark made by St. Paul on the qualifications of a pastor strongly hinted at the first sin of Satan to be insolence. The Apostle wrote in I Timothy 3:6, “He must not be a recent convert, or he may become conceited and fall under the same judgment as the devil.”
In Isaiah 14:12 ff. a description of the pride of the heathen Babylonian king is given in language which many have thought speaks of Satan’s very act of rebellion in desiring that all the worship due God be aimed his way. (This section is akin to Ezekiel 28:11 ff., where the king of Tyre is also upbraided in terms which draw a comparison between his fall and that of Adam and possibly also that of Satan’s expulsion from heaven. Again, we find the sin of pride mentioned.) The very fact that human beings were tempted in the garden of Eden to “become like God” (Gen. 3:5) may be the greatest clue as to the original sin also of the Tempter himself.
Whatever this sin was, it is important for us to maintain that the devil and his fellow evil angels fell away from God after the seven days of creation (when all was good—even the created angel beings), and yet before the fall of Eve and Adam. These fallen angels cannot be converted (Matt. 8:29), for they are facing the eternal fire of punishment (Matt. 25:41). It is our assumption that the remaining holy angels of God are confirmed in their holiness and can no longer fall away, for God has assigned to them responsibility to minister to the believers in Christ who are the heirs of salvation (Heb. 1:14).