The problem is, because the word "lucifer" was capitalized in the King James Version, it was taken for a name, instead of a descriptive word for which it was intended. This is a Latin word, not an English word, so it should not appear in our English Versions of the Bible, unless it was a name, then it should be the same in all languages. If the translators thought this was a name, and wanted to retain the original, it should have been the Hebrew word "heylel." The Latin word "lucifer" was taken from the Latin Vulgate translation of the Scriptures.
The ironic thing is that the Hebrew word "lucifer" is exactly equivalent to the Greek word "phosphoros" which Peter called Jesus in II Peter 1:19, when he said "let the day star arise in your hearts." The definition of "Phosphoros" from Strong's Concordance is: "light-bearing"(Phosphorus), i.e. (spec.) the morning-star.
How did we determine that the word "lucifer" is equivalent to the word "phosphoros?" We consulted the 1828 Webster's Dictionary (the original edition). Under the word "phosphor" we find: n. [Gr. light, from to shine, and to bring. See Phosphorus] The morning star or Lucifer, Venus, when it precedes the sun and shines in the morning. In this sense, it is also written Phosphorus.
The idea that Satan was named "Lucifer" was used by Shakespeare "And when he falls, he falls like Lucifer, never to hope again." So the application of the name Lucifer to the devil is a long-standing one. However, this does not make it accurate.
So why do many Theologians believe that the devil was once named Lucifer? Because the word "lucifer" appears capitalized in the King James Version and therefore it must be a name. It is certainly not the name of the one to which Isaiah 14 is addressed, i.e., the King of Babylon, so since it is capitalized, albeit erroneously, it must be referring to someone else, they say, hence it must be Satan. The word was meant to describe the King of Babylon in a taunt, or parable, in which it was not necessary for everything that was said to be actually descriptive of the individual being taunted. It simply reflects the way the taunted one views himself.
When you read Isaiah 14:12, start with the beginning of the chapter, not in the twelfth verse. Don't we want to keep things in context, so we can understand the real meaning? Look at verse 4. The descriptions that most ministers give of Satan, as they say, "before his fall," come from Isaiah 14:12, which is actually describing the King of Babylon and calling him a man (in verse 16), a ruler (in verse 5), he ruled the nations (verse 6), Thy pomp has been brought down to the grave the worm is spread under thee, and the worms cover thee. (verse 11). What kind of worms could be a threat to a spirit being? This is speaking of a man. This man had children, sons and nephews, etc. (verses 21-22).
And then there is the matter of Ezekiel 28, when taken out of context and started after verse two, which plainly states that it is God's judgment on Tyre. There is really nothing in this chapter to make one think that it is speaking of Satan unless you have read Isaiah 14 and thought it described Satan. Then, some of the attributes given for the King of Tyre might bridge the two Ez. 28 and Isa. 14 together. I believe such a connection to be error.
I'll add more to this one later if the Lord allows.