Several places to start:
1) The KJV does not use the same Old Testament that the Apostles and Christ used: "The LXX helps explain what Paul might have meant by ‘all Scripture.’ As previously mentioned, this is the version Paul most often quotes. And in some cases the claims of the New Testament theologically depend on the peculiarities of the LXX.
For instance, Hebrews 10:5 quotes Psalm 40:6 as a messianic prophecy:
Therefore, when He comes into the world, He says, ‘sacrifice and offering Thou hast not desired, but a body Thou hast prepared for Me.’
The author has directly quoted from the LXX Psalter. A quick turn to our modern Bibles will confirm that the Hebrew text reads:
Sacrifice and meal offering Thou hast not desired; My ears Thou hast opened.
Based on the Hebrew text, the author of Hebrews has not only misquoted the passage, but has made his mistaken citation a central part of his argument. Only the rendering of the LXX justifies this as a Messianic passage. Did the author of Hebrews get it wrong? Was it an inspired mistake?
In Acts 7:14 St. Stephen relates the story of the Israelite nation, and refers to seventy-five people who traveled from Canaan to Egypt in the emigration of Jacob's family. This is not what Genesis 46 states in our Bibles, where it catalogues seventy sojourners. But the LXX lists seventy-five people, confirming St. Stephen's account, with the differences accounted for by the grand- and great-grandchildren of Joseph (Gen 46:20-22).
Most importantly, it is only in the LXX that Isaiah's prophecy of the Virgin Birth makes its bold appearance (Is 7:14). The Hebrew text uses the word ‘woman’ (‘marah’) instead of ‘virgin’ (‘parthenos’). In their earliest confrontations with Christians, Jews objected most strongly to this verse being used to support Jesus' Messiahship. The Jews claimed that Isaiah was prophesying of King Hezekiah and he knew nothing of a miraculous virgin birth. The Septuagint, they said, had been tampered with. The early Christians responded by claiming that it was not they, but the Jews who had cut passages out of the Hebrew text out of envy. (Justin Martyr, Trypho, 71-73)
If we agree with the ancient Jews that the LXX translation was a faulty translation, then why is such a substandard text part of Holy, Inspired Scripture? Doesn't the New Testament suggest that the LXX was considered not just trustworthy, but even preferred by the Apostles? This is not out of harmony with the testimony of the Early Church, which regarded it as a sound and inspired translation.
As a Bible believing Christian, facing this dilemma was not easy. I felt that by trying to honestly grapple with textual issues, I was questioning the authority of God's Word. This is not at all what I intended. I simply wanted integrity in my Christian faith. With time, as I struggled through some of these facts, I realized I needed to come to Scripture on its own terms, not on my expectations as a twentieth century Westerner. This desire for integrity aided me as I swallowed hard and proceeded to study the canon of the Old Testament." (
http://www.kalvesmaki.com/LXX/)
Second, American KJV cultists HATE HATE HATE HATE HATE the "Apocrypha". The original KJV included these books. Have him explain how an "infallible" translation could start out with such an enormous "error".
Third, in the introduction to the KJV, the translators/editors wrote the following: "Now in such a case, doth not a margine do well to admonish the Reader to seeke further, and not to conclude or dogmatize upon this or that peremptorily? For as it is a fault of incredulitie, to doubt of those things that are evident: so to determine of such things as the Spirit of God hath left (even in the judgment of the judicious) questionable, can beno lesse then presumption. Therfore as S. Augustine saith, that varietie of Translations is profitable for the finding out of the sense of the Scriptures: so diversitie of signification and sense in the margine,"
In addition, consult the following:
http://www.kjvonly.org/james/may_great_inconsistency.htm
http://www.baptistdeception.com/kjv-only-deception/#.V9lBpFsrLcs
https://bible.org/article/why-i-do-not-think-king-james-bible-best-translation-available-today
