Charles Spurgeon Commentary Isaiah 40:5

"and the glory of Jehovah shall be revealed, and all flesh shall see it together; for the mouth of Jehovah hath spoken it." — Isaiah 40:5 (ASV)

And the glory of the LORD shall be revealed, and all flesh shall see it together: for the mouth of the LORD hath spoken it.

And, since he has spoken it, it must come to pass. Hath he said, and shall he not do it? With him, to say anything is to will its accomplishment.