29 Furthermore, we must form a conclusion why it is that the words cited above, Therefore thus says the Lord, are followed by But the Lord shall slay you, and as to the meaning of the next sentence, But my servants shall be called by a new name, which shall be blessed upon earth. There can be no doubt that both Therefore thus says the Lord, and afterwards But the Lord shall slay you, prove that it was the Lord Who both spoke, and also purposed to slay, Who meant to reward His servants with that new name, Who was well known to have spoken through the prophets and was to be the judge of the righteous and of the wicked.
And thus the remainder of this revelation of the mystery of the Gospel removes all doubt concerning the Lord as Speaker and as Slayer. It continues:— But My servants shall be called by a new name, which shall be blessed upon earth. Here everything is in the future. What then is this new name of a religion; a name which shall be blessed upon earth? If ever in past ages there were a blessing upon the name Christian, it is not a new name. But if this hallowed name of our devotion towards God be new, then this new title of Christian, awarded to our faith, is that heavenly blessing which is our reward upon earth.
Source: On the Trinity (New Advent)