As we know there are two witnesses that will prophecy at the beginning of the 7 year tribulation period for 1,260 days. Revelation 11:3. One of those witnesses is Elijah as per Malachi 4:5-6.
There are a number of reasons why John is obviously the second witness.
We know that the bible interprets itself and all the information we need is written inside its covers.
Revelation 10:2 And in His hand was a little scroll having been opened. And He set his right foot on the sea and His left foot on the land,
3 and called out with a loud voice, as a lion roars. And when He called out, seven thunders proclaimed their sayings.
4 And when the seven thunders spoke, I was about to write. But I heard a voice from the sky saying: “Seal up the sayings of the seven thunders,” and, “you should not write them.”
The book of revelation is called the book of revealing. Revealing of who Jesus is and then details about what is going to happen in the End Times using many Old Testament scriptures as guides.
However, Jesus tells John not to write down what he heard the 7 thunders say in Revelation 10:-2-4. Up until this and subsequent chapters John writes down everything that he saw. So the question is why did Jesus stop John from writing it down or even let him hear what was said?
Well Revelation 10:11 states:- And they said to me: “It is necessary for you to prophesy again against many peoples, nations, tongues, and kings.”
So when is he going to prophecy again against many peoples, nations, tongues and kings?
John did it the first time by writing Revelation down as it is a dire warning to not just Israel and the Middle East but to the whole world.
John never wrote another letter or personally prophesied what was in the 7 seven thunders as far we know. He died some 5 years after authoring the book of Revelation.
Then in Ezekiel 2:9-10,3:1-3 we have the following picture which parallels Revelation 10 and the little scroll written on both side as per Revelation 5:1.
Revelation 10-11:13 is parenthetical portion of scripture that adds extra information to the story line which is not part of the sequences like the 7 seals, 7 trumpets and 7 bowls. For example that the witnesses will prophesy 1,260 days stated in Revelation 11:3 .
9 Now when I looked, there was a hand stretched out to me; and behold, a scroll of a book was in it.
10 Then He spread it before me; and there was writing on the inside and on the outside, and written on it were lamentations and mourning and woe.
Ezekiel 3:1 Moreover He said to me, “Son of man, eat what you find; eat this scroll, and go, speak to the house of Israel.”
2 So I opened my mouth, and He caused me to eat that scroll.
3 And He said to me, “Son of man, feed your belly, and fill your stomach with this scroll that I give you.” So I ate, and it was in my mouth like honey in sweetness.
4 Then He said to me: “Son of man, go to the house of Israel and speak with My words to them.
So we see that Ezekiel also had a scroll written on both sides and he had to eat it. It was sweet but as we see later it became bitter as he was sent to announce God’s judgment & sifting of Israel & the surrounding nations.
Then, finally in Revelation 1:9 John states this:-
Revelation 1:9 I, John, both your brother and companion in the tribulation and kingdom and patience of Jesus Christ, was on the island that is called Patmos for the word of God and for the testimony of Jesus Christ.
How can John by the companion of the churches that he is writing to in the Tribulation when the tribulation was a future event and the tribulation never happened in John’s life time?
The only way this can happen is if he is resurrected at the beginning of the Tribulation and he is our companion as he will prophecy what he heard the 7 thunders say and any other words from the Lord.
What did the Early Church Believe?
Early Christian writers understood from this passage that John would be raised from the dead in the last days to proclaim the message of the seven thunders and the contents of the little scroll. (See Hippolytus, App. xxi). To “prophesy again against…” implies John’s once again standing in opposition to these peoples, just as Ezekiel did. This is based on the same set of circumstances stated in Ezekiel (1:26-3:11)
The only logical and common sense answer is that a New Testament prophet who heard the 7 thunders prophecy and told by Jesus he must prophesy again against many peoples, nations, tongues, and kings is John the Apostle. We will have an Old Testament prophet Elijah being resurrected as the second witness to prophecy for 1,260 days with John.