Italicized words and phrases are common in the Bible. They are inserted to make the passage more understandable. Here’s an example from Romans 5:18 (KJV): “Therefore as by the offence of one judgment came upon all men to condemnation; even so by the righteousness of one the free gift came upon all men unto justification of life.” - The words judgment and gift are not in the source text (textus receptus for the KJV) . However, their absence does not remove the message being conveyed: Offence of one – condemnation on all; righteousness of one – justification for all.

There is one verse in which italics seem to be used not for clarification but for a different reason. The verse is I John 2:23: “Whosoever denieth the Son, the same hath not the Father: (but) he that acknowledgeth the Son hath the Father also.” The TR reads: “πᾶς ὁ ἀρνούμενος τὸν υἱὸν οὐδὲ τὸν πατέρα ἔχει, ὁ ὁμολογῶν τὸν υἱὸν καὶ τὸν πατέρα ἔχει.” The second part of the verse (italicized in the KJV) is clearly IN the source text.

This leads to the question “Why did the translators use italics?” The probable reason may be found by tracing back to the Byzantine manuscripts, the primary source used by Desiderius Erasmus and his successors to develop the TR. The representative text reads: “Πᾶς ὁ ἀρνούμενος τὸν υἱὸν οὐδὲ τὸν πατέρα ἔχει.” That’s the whole verse; the last part is not included in Byzantine manuscripts. It would appear the KJV translators were not quite sure what to make of the discrepancy. Perhaps they couldn’t decide whether to follow the text or the underlying manuscripts, so they just used italics.

The early manuscripts (some of which were not available until the nineteenth century) disagree with the Byzantine and match the TR.

The translators of the New King James Version (NKJV) apparently didn’t have as much trouble with their decision; their translation reads: “Whoever denies the Son does not have the Father either; he who acknowledges the Son has the Father also.” No italics.