Question: "What was the first sin?"
Answer: The first sin must have preceded Adam’s and Eve’s choices to sin, since the tempter in the garden was already evil before he talked to Eve. There is no biblical evidence for any sin having preceded Satan’s.
The occasion of Satan, or Lucifer, being cast out of heaven is described in Isaiah 14:12: “How you have fallen from heaven, O star of the morning, son of the dawn! You have been cut down to the earth, You who have weakened the nations!” (NASB). Some versions, such as the New King James, use the word Lucifer instead of the phrase star of the morning, and that is where Lucifer’s name comes from.
It’s important to note that Lucifer’s sin preceded humanity’s sin. We know from Romans 5:12–21 that all men and women throughout history (except, of course, for Jesus) have been sinners as a result of Adam’s sin. Since Lucifer’s sin preceded that of Adam, it also preceded that of the rest of humanity.
Furthermore, Lucifer’s sin apparently preceded that of the rest of the angels who fell and, we assume, were corrupted into demons. In Revelation 12:4, we read that the tail of the dragon (who represents Satan or Lucifer) “swept a third of the stars out of the sky and flung them to the earth.” Then, in verse 9, we read that “the devil, or Satan, . . . was hurled to the earth, and his angels with him.” This passage speaks of the fall of Satan, who took one third of the holy angels with him in his rebellion. Thus, Lucifer’s sin preceded both mankind’s and the other angels’.
Finally, the Bible does not tell us about any other creatures capable of sinning against God. Angels and humans are the only beings who are morally responsible and therefore able to sin. It is reasonable to conclude that Lucifer’s sin was indeed the first sin, as both human sin and angelic sin came after his fall. There seems to have been no sin prior to Lucifer’s.