In Revelation 11, during the Great Tribulation and the Day of the Lord, John describes the Two Witnesses as having God-given power to witness and, if need be, call fire down from heaven to destroy their enemies. While they preach God's last warning to the human and demonic powers of earth just before the final Trumpet sounds, God gives them supernatural protection.
Before the start of their 3½-year commission, the Two Witnesses will already have been witnessing by their example and through their preaching. They will already be producing good works. God will inspire them to utter a specific message directed at the descendants of Israel and the rest of the world, indicting the world of sin.
Revelation 11:3 says:
And I will give power to my two witnesses, and they will prophesy one thousand two hundred and sixty days, clothed in sackcloth.
The word "power" is not in the Greek text, which simply reads, "I will give to my two witnesses, and they shall prophesy. . . ." Paraphrased, God says, "I will grant to My Two Witnesses the right or power of prophesying during the time specified." Translators must add a word like "power," "privilege," "opportunity," or "boldness" to complete the sense in English.
The meaning is not that God would send two witnesses to prophesy, but that they are existing witnesses who receive additional gifts and powers. During that time God will give them the privilege and the strength to proclaim the truth that they will be commissioned to communicate as His "witnesses" to mankind.
The phrase "and they will prophecy" does not necessarily mean that they would predict future events, but that they would proclaim the truth as God had revealed it. The indication here is that the Two Witnesses would publicly preach or maintain the truth before the world.
God promises protection to those who obey Him, provided it is His will. Some of the faithful are given the spiritual strength to be martyred, and others are protected from such unpleasantness. God decides for His own purpose how He wants us to represent Him. Revelation 11:5 describes part of God's witness protection program, "If anyone wants to harm [the Two Witnesses], fire proceeds from their mouth and devours their enemies." Christ is reliable and true. Not a hair of our head is affected without His approval.