Rather, “whomsoever.” Paul is quoting these words from Ex. 33:19 to emphasize his point that it is for God to decide who are to be the recipients of certain favors. It is not for man to dictate to Him.
I will have mercy.
The quotation is from Ex. 33:19. The words were spoken to Moses in connection with his request to see God’s glory. The issue is not one of personal salvation, but one of God’s right to show certain favors to whom He will. The fact that God does not reveal to us His glory in the remarkable way He did to Moses is no evidence of injustice. “God is too wise to err, and too good to withhold any good thing from them that walk uprightly” (SC 96; see Ps. 84:11).