For so hath the Lord commanded us, saying, etc. - These and other predictions must have been long before this brought vividly home to Paul’s mind in connection with his special vocation to the Gentiles.
I have set thee - that is, Messiah; from which Paul inferred that he was but following out this destination of his Lord, in transferring to the Gentiles those “unsearchable riches” which were now by the Jews rejected and despised.