Promise - The Lord's guidance (Isaiah 42:6)

I the LORD have called thee in righteousness, and will hold thine hand, and will keep thee, and give thee for a covenant of the people, for a light of the Gentiles; To open the blind eyes, to bring out the prisoners from the prison, and them that sit in darkness out of the prison house. I am the LORD: that is my name: and my glory will I not give to another, neither my praise to graven images.
(Isa 42:6-8 KJVR)

Called thee - To declare my righteousness, or faithfulness. With - hold - Will give thee counsel and strength for the work. Give thee - To be the mediator in whom my covenant of grace is confirmed with mankind. The people - Of all people, not only of Jews but Gentiles. A light - To enlighten them with true and saving knowledge.