Isaiah now quotes the Lord as having endorsed his words. The Lord has said that he called Isaiah to perform his great mission. The Book of Mormon version also has the words "to declare" the things which Isaiah had been teaching them. Now Isaiah speaks directly to the people and bears his testimony.
16. Come ye near unto me; I have not spoken in secret; from the beginning, from the time that it was declared have I spoken; and the Lord God, and his Spirit, hath sent me.
Isaiah wants the people to know that the person who dispatched him on this mission was none other than the Redeemer, the Holy One of Israel.
17. And thus saith the Lord, thy Redeemer, the Holy One of Israel; I have sent him, the Lord thy God who teacheth thee to profit, who leadeth thee by the way thou shouldst go, hath done it.
The Lord's lamentation concerning Israel is that they have not been obedient to his commandments. If they had been obedient they would have enjoyed tremendous blessings, and it would not have been necessary to scatter them all over the face of the earth.