Italicized words were first used in 1560 when an edition of the Bible, known as the Geneva Bible, appeared. This Bible had been prepared by the Protestant reformers in Geneva and was translated directly from the original Hebrew and Greek. In it, there were words that had to be added in English to make the meaning plain, although they were not necessary in the original idioms. No language can be translated word for word. The translators, then, distinguished such necessarily added words by italicizing them. The Geneva Bible became the most popular Bible of its time.
By the beginning of the seventeenth century, there were three versions of the Bible in England, but these translations were by no means correct, and as time went on, the meaning of some of the English words changed. The need for a better translation became apparent, and from this need came the most used version even today, the King James or Authorized Version. King James I gave the task of translation to a group of 54 translators, and in their translation, they followed the lead of the Geneva Bible translators and made use of italics for added words.
In most cases, italicized words clarify the meaning of certain phrases. However, because these translators were not necessarily inspired by God in their work (though some would claim so), they made mistakes.
For instance, the LORD tells Moses at the burning bush that His name is I AM (Exodus 3:13-14). About 1,500 years later, when Jesus is about to be arrested in the Garden of Gethsemane, He tells them—in the King James and many other versions—"I am He" (John 18:3-8). The "he," however, is in italics; that means Jesus did not say it! He really said them, "I AM," identifying Himself as the LORD who spoke to Moses, the very God of Israel! No wonder the multitude fell back!
Another mistaken use of added (italicized) words occurs in Revelation 20:10. Please see our Question and Answer on this subject.
The lesson, then, is that, since these words are not original to the Bible, they should be regarded with caution. Most of the time, the translators supply the proper words for the original sense to be understandable in our modern English. Yet, as with all human endeavor, there are times when errors occur, so no italicized words should be the basis for determining doctrine.