The statement in 53:10 that “God shall prolong His days”, as is rendered in most English translations (28 of 31) could be understood as God extending Christ’s lifespan: 23 translations speak of prolonging His days; 1 speaks of lengthening His days; 2 speak of Him “enjoying a long life”; 1 speaks of Him “living a long life”; and 1 speaks of Him “living, to see His descendants”. There must be good reason why so many are translated this way; as if He was a mere mortal, but it is not clear why. Granted His earthly life as the incarnate Messiah was ended by His crucifixion; but Christ is an eternal Member of the Triune Godhead, the very agent of creation, itself! There are examples in the Bible of God prolonging men’s lives, such as in 2^nd Kings 10:1-6, where God added 15 years to Hezekiah’s life in answer to his prayer; but this is quite different from that. Insight from the New Testament confirms that these words of Isaiah was a prophecy of the suffering, death, and resurrection of the true Messiah - Christ, the Son of God, an eternal member of the Holy Trinity. But his choice of words about “prolonging His days” or “He will enjoy a long life”, as the NLT renders it, could be misleading to some. The temptation to reword this phrase in verse 10 to remove that risk was strong, But His Word is “God-Breathed”; and no one should take it upon themselves to alter the sense of what God’s word is saying – even with the best intentions. And so it remains as they translated it, except for the purple font treatment that is unique to this Purple Letter Edition. Even the bold/no-bold font ‘device’ usually used to distinguish between references to God and His Son is not used here, because it is still not clear who is prolonging whose days.

The Bible commentaries on Isaiah 53:10 are fairly silent on this issue; but a few do ‘touch lightly upon it’; and those excerpts are copied below, with a few underscores added:

Jamieson-Fausset-Brown Commentary - “prolong … days—also esteemed a special blessing among the Jews (Ps 91:16). Messiah shall, after death, rise again to an endless life (Hosea 6:2; Romans 6:9).”     Gill’s Exposition - “he shall prolong his days - live long, throughout all ages, to all eternity; though he was dead, he is alive, and lives for evermore; lives to see all the children that the Father gave him, and he has gathered together by his death, when scattered abroad, and see them all born again, and brought to glory. Some connect this with the preceding clause, "he shall see a seed that shall prolong its days"; for Christ will never want issue, his church will never fail, his seed will endure forever, Psalm 89:29. So the Targum (an ancient Aramaic interpretation of the Hebrew Bible), paraphrases the words of Christ and his seed, "they shall see the kingdom of their Messiah; they shall multiply sons and daughters; they shall prolong their days:'' and so Aben Ezra (a distinguished Jewish scholar and biblical commentator (1092-1167)) says these words are spoken of the generation that shall return to God, and to the true religion, at the coming of the Messiah.            Benson Commentary –“He shall see his seed — His death shall be glorious to himself and highly beneficial to others, for he shall have a numerous seed of believers, reconciled to God, and saved by his death. He shall prolong his days — He shall be raised to immortal life, and live and reign with God for ever.”     Matthew Poole’s Commentary - “He shall prolong his days -  he shall be raised to immortal life, and shall live and reign with God for ever; he shall die no more, (Rom.6:9), and of his kingdom there shall be no end, (Luke 1:33).”  MacLaren’s Expositions - “What a contrast there is between platitudes about the spirit of the nation rising transformed from its grave of captivity {which was only very partially the case}, and the historical fulfilment in Jesus Christ! Here, at any rate, hundreds of years before His Resurrection, is a word that seems to point to such a fact, and to me it appears that all fair interpretation is on the side of the Messianic reference.”

Here is another possible explanation for why so many translate Isaiah’s words as “God shall prolong His days”: Isaiah’s vision and prophecy of the events of Christ’s crucifixion and resurrection, penned so long before they occurred, may reflect a sort of ‘duality’ of views: one of a king in the lineage of David that the Jews were hoping would arise and deliver Israel from its oppressors – an earthly king the Jews hoped God would prolong the days of; and that of Christ, the true and eternal Messiah - the One that the Jews of Isaiah’s day were not looking for or expecting. When you consider that (1) King David had reigned about 300 years before Isaiah’s prophecy, and (2) Isaiah penned it about 700 years before the birth, Jesus Christ, Israel’s true Messiah; and (3) that in Jewish eschatology, the messiah is a future Jewish king, a descendant of David, who is expected to be anointed with holy anointing oil and rule the Jewish people during the Messianic Age and world to come – then it is plausible that this statement in verse 10 is referring to prolonging the life of that earthly king, the deliverer the Jews were longing for. And if this is so, then the clause should be rendered here in the RFP Bible as “God shall prolong his days”, (with no purple font or capitalization treatment for “his”). Dual references are not uncommon in Scripture; and this may be one of the more subtle ones. There is some evidence that both the subject and the object of this clause are both Christ; and should be rendered as Christ prolonging His Own days.