Then I saw another mighty angel coming down from heaven, wrapped in a cloud, with a rainbow over his head, and his face was like the sun and his legs like pillars of fire. He had a little scroll open in his hand. And he set his right foot on the sea and his left foot on the land.
This angel reveals things to John and then says he cannot tell others. The only real reason I can see is that with this information, we would know something that would interfere with the second coming of Christ. The only people who could change that are Christians.
This entity is described as an angel; who is he? He has a rainbow above his head. I doubt that has anything to do with LGBT rights or Greenpeace. The rainbow was a symbol of God's promise that he would not destroy man with a flood again.

 He was wrapped in a cloud, and his face was like the sun. That is a picture of the power of Jesus. His legs were like pillars of fire. Fire being a picture of the Holy Spirit, one foot was in the sea and the other on the land. That is everywhere, like the Holy Spirit. I expect this is the only picture we ever see of the Holy Spirit. The Holy Spirit then says in Revelation 10:5–6, And the angel whom I saw standing on the sea and on the land raised his right hand to heaven and swore by him who lives forever and ever. Who created heaven and what is in it, the earth and what is in it, and the sea and what is in it, that there would be no more delay. The Holy Spirit will only be saying this to the Father. When you look at this verse quickly, you just think sky, land, and sea. However, the word earth in Greek refers to the place. The place of the earth has the sky and the sea in it, the atomic world. What are these places that are being talked about? The sea is often a description of the people, but in this case, it will be all the creatures that exist that are atom-based beings. All the beings in the first, second, and third heavens. All their interactions with each other, the powers they have, and the rules they must abide by. The heaven will be the original heaven, which is profound because that suggests that the original heaven was made and that there is another place beyond that. That leads us to places beyond our comprehension.

The Holy Spirit wrote the bible or inspired men to put down the right words. If he is revealing things to John that are being done wrong, he will most likely be talking about the gospel and the Bible. His spirit dwells within us, so the next most likely candidate will be that. His spirit dwells in us and his spirit inspired the Bible are so intertwined that if you are talking about one, you will be talking about the other.