Commentaries:
This may also be part of the message of the Two Witnesses. They will preach comfort to Jerusalem—that the end is about to come, that she's been repaid for her sins, but a time is coming that will be far better for her. They will also prepare the way of the Lord, an obvious aspect of their ministry. In addition, they will proclaim that the Day of the Lord is coming, a time when all flesh is grass—when many will be simply wiped out for their sins. They will also preach a message to the church, leading its part in bringing good tidings of the coming Kingdom of God and giving them encouragement to do it with strength and boldness in Judah (evidently where most of them are at this late date). Finally, of course, they will boldly announce the return of Jesus Christ and the establishment of His government.
This passage, in a way, encapsulates the witness or the testimony of the Two Witnesses—to the world and to the church. Theirs will be true evidence that brings a conviction. One could say that the Two Witnesses are the two star witnesses of an end-time trial in which God judges that the world must be punished, that He must send His Son back, and that His must rule mankind. The Two Witnesses will give their evidence, and God will pass judgment.
Richard T. Ritenbaugh
The Two Witnesses (Part Three)
Other Forerunner Commentary entries containing Isaiah 40:8:
Numbers 23:19
Matthew 16:24-27