27 To ensure that our explanation of the passage shall be complete and certain, I cite the words in full:— Therefore thus says the Lord, Behold, they that serve Me shall eat, but you shall be hungry, behold, they that serve Me shall drink, but you shall be thirsty, behold, they that serve Me shall rejoice with gladness, but you shall cry for sorrow of your heart, and shall howl for vexation of spirit. For you shall leave your name for a rejoicing unto My chosen, but the Lord shall slay you.
But My servants shall be called by a new name, which shall be blessed upon earth; and they shall bless the true God, and they that swear upon the earth shall swear by the true God. There is always a good reason for any departure from the accustomed modes of expression, but novelty is also made an opportunity for misinterpretation. The question here is, Why, when so many earlier prophecies have been uttered concerning God, and the name God, alone and without epithet, has sufficed hitherto to indicate the Divine majesty and nature, the Spirit of prophecy should now foretell through Isaiah that the true God was to be blessed, and that men should swear upon earth by the true God.
First, we must bear in mind that this discourse was spoken concerning times to come. Now, I ask, was not He, in the mind of the Jews, true God, Whom men used then to bless, and by whom they swore? The Jews, unaware of the typical meaning of their mysteries, and therefore ignorant of God the Son, worshipped God simply as God, and not as Father; for, if they had worshipped Him as Father, they would have worshipped the Son also. It was God, therefore, Whom they blessed and by Whom they swore.
But the prophet testifies that it is true God Who shall be blessed hereafter; calling Him true God, because the mysteriousness of His Incarnation was to blind the eyes of some to His true Godhead. When falsehood was to be published abroad, it was necessary that the truth should be clearly stated. And now let us review this passage, clause by clause.
Source: On the Trinity (New Advent)