Revelation 1:9 says:

"I, John, your brother and fellow participant in the tribulation and kingdom and perseverance in Jesus..."

This clearly shows the tribulation was already happening when John wrote. He calls himself a "fellow participant" in it, not a future observer. He doesn't say it will come, he says he's in it, alongside the churches he's writing to.

This matches what Jesus warned about in Matthew 24:9:

"Then they will hand you over to tribulation and kill you, and you will be hated by all nations because of My name."

John was living through that exact persecution. He was exiled to Patmos because of the word of God and the testimony of Jesus (Rev. 1:9). That's not a future event, it's a present reality in the first century.

If the tribulation was already active, then placing it thousands of years later contradicts what the text and John himself plainly say.

And for those that try to change the meaning of tribulation, the word used in revelation 1:9 is the same word in all of revelation. Tribulation and great tribulation have the same meaning in Revelation (Rev. 1:9, 2:9, 2:10, 2:22, 7:14).