"And I appeared unto Abraham, unto Isaac, and unto Jacob, by the name of God Almighty, but by my name JEHOVAH was I not known to them." (Exodus 6:3, KJV)
"JEHOVAH" is the translation of יהוה with the Masoretic vowel points. These vowel points were put in place by the Masoretes many centuries after the composition of the Hebrew Bible. It is commonly believed that the Jews in the Second Temple period took the vowel points of "Adona" (Lord) and placed them in יהוה.
Many scholars today believe that the original pronunciation of יהוה was closer to "Yahweh". This is a theory based on how some writers in the cognate languages and Greek pronounced the name (e.g. Clement of Alexandria, The Stromata, Book V, 6). These other languages had vowels written before the Masoretes gave vowel points to the Hebrew, and it appears that some early writers in these languages wrote the name as something resembling "Yahweh".
However, there is more to the story. Our Lord's Hebrew name "Yehoshua" (יהושׁע) means "Jehovah saves" and is derived from יהוה. This Hebrew name became "Yeshua" in Aramaic and "Ἰησοῦς" (Iesous) in Greek. As clearly seen, by the time of our Lord Jesus the part of "Yehoshua" (יהושׁע) which corresponds with יהוה was pronounced like the first two syllables of "Jehovah". Whether or not the original pronunciation of יהוה was "Yahweh", Jews pronounced the name as "Jehovah" by the first century. And neither our Lord Jesus nor the New Testament writers indicated any dissatisfaction with this pronunciation.
Other Hebrew names were also derived from compounding יהוה (or a portion of it) with another Hebrew word: e.g. Jehoshaphat, Jehoiada, Jehoram, Jehoiakim, etc. The way these names have been pronounced throughout history suggests to us that יהוה was pronounced like "Jehovah". In any case, the fact that the psalmist was comfortable using the short-form “Jah” (Psalm 68:4) indicates that the Jews were not particularly legalistic about the pronunciation of the Divine Name. Pious Jews did not even utter the sacred name.
The theory of the pronunciation as "Yahweh" is a theory at best. In any case, the KJV is a translation of the Hebrew Masoretic text, which gives the vowels of "Jehovah". As such, the KJV is a faithful translation of the preserved Hebrew text.
Lasty, some critics despise the hard “J” sound in “Jehovah,” but many Hebrew names (e.g. Jesus, Joseph, John, James, Jude, Judas, Jeremiah, Jonah) are pronounced in English with the hard “J” despite them not having the hard “J” in Hebrew or Greek. It would be hypocritical to insist that one must pronounce יהוה as “Yahweh” while permitting the Anglicized pronunciations of other Hebrew names (including “Jesus,” which contains the name “Jehovah.”).
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