God says there have been many witnesses already extant in the world. The apostle Paul, in the book of Colossians, states that the final witness and warnings of the Word had already be preached "to every creature in heaven" in the first century A.D. (Colossians 1:23).
The prophets of God came with another type of final warning and witness. A prophet would tell Israel, "Don't do this. Don't do this." The prophets and preachers of righteousness have their place but they are NOT the ultimate witness. The book of Revelation states that in the end time just ahead of us there are going to be two witnesses who will warn the world of what will happen if they do not repent (Revelation 11:3).
For three and a half years Revelation's two witnesses are going to be invincible. No one will be able to kill them during their ministry of final warnings just before the return of Christ. At the end of their forty-two month period of service, however, they are going to be allowed to be killed. Their bodies will be unburied for three days until they are miraculously resurrected. Revelation, however, reveals still other witnesses from God.
In Revelation 6 we read of the sixth seal. It is a time, in the near future, when the sun will darken and the moon will look like the color of blood. The sky over the earth will appear as if it is being scrolled back (Revelation 6:12 - 14). When the moon turns like blood, the sun grows dim, the heavens roll back, and a giant earthquake takes place - that's a pretty big final witness from God, wouldn't you say?
Another witness is the trumpet plagues. God opens up one after another of these plagues and each of them constitutes a warning to repent of sin. After the seventh angel sounds his trumpet, loud voices are heard in heaven with a message that is especially noteworthy.