What was Satan's sin?

[Timothysis]

“non serviam” were Satan’s words according to Apostolic Tradition (not in scripture but as valid to Catholics as scripture). The meaning is “I will not serve” a God who will sacrifice His only begotten Son for an inferior race such as mankind.

*"Biblical references to the fall of Lucifer or Satan do not quote these words.

However, in the Latin Vulgate, Jeremiah laments that the people of Israel speak “Non serviam” to express their rejection of God.

The words became a general expression of the basic manner of rejecting God, such that it would apply to the fall of Lucifer. The words have thus been attributed to Lucifer."*