He pleads with them to rally around their Lord and Savior.
13. Mine hand hath also laid the foundation of the earth, and my right hand hath spanned the heavens. I call unto them and they stand up together.
In this verse the Lord reminds his rebellious people who he really is. He laid the foundations of both the heavens and the earth. He organized the myriads of intelligences and vast quantities of primal matter to come together according to certain patterns and designs. When he spoke to them and commanded them, they literally stood up in their places according to his word and became the marvels of the cosmic universe that we now see all around us.
14. All ye, assemble yourselves, and hear; who among them hath declared these things unto them? The Lord hath loved him; yea, and he will fulfil his word which he hath declared by them; and he will do his pleasure on Babylon, and his arm shall come upon the Chaldeans.
Now Isaiah discontinues quoting the Lord and tells the people to assemble themselves together. He asks who is it that has been declaring these things to them. The answer of course is Isaiah himself, who has been proclaiming these great prophecies to the people. Isaiah wants the people to know that the Lord has loved him. And then there is a phrase which is not in the Bible but is in the Book of Mormon version and it says, "and he will fulfil his word which he hath declared by them," that is, by the prophecies that Isaiah has pronounced.