I ... us - The change of number indicates the Trinity (compare Gen 1:26; Gen 11:7). Though not a sure argument for the doctrine, for the plural may indicate merely majesty, it accords with that truth proved elsewhere.
Whom ... who - implying that few would be willing to bear the self-denial which the delivering of such an unwelcome message to the Jews would require on the part of the messenger (compare 1Ch 29:5).
Here am I - prompt zeal, now that he has been specially qualified for it (Isa 6:7; compare 1Sa 3:10, 1Sa 3:11; Act 9:6).