Christ is quoting from Isaiah and applying the words to a specific time frame. It is post-gentile receipt of the Book of Mormon, post-delivery of that book to the remnant, and post-opportunity for gentiles to repent and know of the true points of His doctrine. When that happens, the Lord will be freed up to fulfill the covenants of the Father.
When the Father's covenants are in being fulfilled, "kings shall shut their mouths." That is, the noble of this world will not know what to say. They will be at a loss of words because of the Father's acts.
Things that haven't been "told them" will take place, and they will not understand.
Things that they never had taught to them will unfold, and they cannot comprehend, cannot get their hands around it all. It will dumbfound them.
Even when people who understand that the events are according to the Father's plan, and the Lord's covenant, they will not be able to believe it.
Too much! Too distressing! Too unexpected! Too great to take in! It will be confusion and distress, and the idea that God is behind it all will be unbelievable to them. (Isa. 52: 15.)
Their plans for managing the world will be dashed and end. Their great investments will be lost.The control they imagined they had as "kings" will fade to dust. (Hag. 2: 22.)
How can such splendor, such great and masterful arrangements, such glory in mankind become nothing? How can it all fall to the dust? (Rev. 14: 7-8.)
It will be "great and marvelous" because it shows the Father's power and might. But it will be inconvenient and distressing, unbelievable and terrible. (Malachi 4: 1.)
What is coming will leave proud men speechless and believing people vindicated. Everything will change.
Once again we see the tremendous relevance of Isaiah's words. You can search for language to capture our times and find no-one who has given phrases that capture this moment better than Isaiah. Even the Lord finds his words appropriate to quote as His own. Words of inspiration, given Isaiah by the Lord, become Isaiah's, then are taken by the Lord and used again as His. The symmetry of this chiasm is a reminder of how closely the Lord works with those He inspires. It is "His voice" even when the message comes from His servants. Who has ears to hear?