In the vision, John saw a beast come out of the sea. He saw another beast come out of the earth. They were like the four animals Daniel saw in a vision recorded in Daniel chapter 7.
In Daniel’s vision, beasts were signs of human governments. Thrones were signs of authority and horns were signs of power. This was true for John’s vision as well. The first beast was a sign of a powerful ruler or government. The second beast was a sign of a person or group that supported them.
In Revelation 16:13 the second beast was also called a false prophet. The devil was the dragon who gave these beasts power over people. The first beast was allowed to rule for 42 months. The second beast made people worship the first beast and be marked in a certain way.
This was the opposite of when God’s servants were marked by God’s seal. The mark of the beast from the sea was the number 666. All of this was like something happening in John’s time in the lands controlled by Rome.
Authorities and rulers made people obey and worship Rome and the emperor. Those who refused to worship the Roman ruler Caesar were treated badly or killed.