The term Lucifer is apparently the title or name of the personage in the pre-earthly existence who is now referred to as Satan or the devil. The fact that Isaiah refers to Lucifer and his role in the pre-earthly existence would seem to indicate that the Old Testament prophets were acquainted with the doctrine of a pre-earthly existence. It may be that Isaiah and others of the ancient prophets had access to the writings of Moses that are not in our present Old Testament but which were revealed anew to the Prophet Joseph Smith in December 1830. These writings of Moses include the following statement concerning Lucifer and his role in the pre-earthly councils:
Wherefore, because that Satan rebelled against me, and sought to destroy the agency of man, which I, the Lord God, had given him, and also, that I should give unto him mine own power; by the power of mine Only Begotten, I caused that he should be cast down;
And he became Satan, yea, even the devil, the father of all lies, to deceive and to blind men, and to lead them captive at his will, even as many as would not hearken unto my voice. (Moses 4:3-4.)