And I saw when the Lamb opened one of the seals, and I heard, as it were the noise of thunder, one of the four beasts saying, Come and see. And I saw, and behold a white horse: and he that sat on him had a bow; and a crown was given unto him: and he went forth conquering, and to conquer. (Revelation 6:2-3).
This is a description of the 1st of the 4 deadly horseman of the Apocalypse. It is a description of Emperor Constantine going forth to conquer the entire Roman world.
All the great prophecies in the Book of Revelation began to be fulfilled with the reign of Emperor Constantine. Before the battle of the Milvian Bridge outside Rome in 312 A.D., Constantine claimed to have seen a cross in the sky with this inscription: IN HOC SIGNO VINCES or IN THIS SIGN CONQUER. He immediately had the Roman eagle replaced by the cross on his battle standards.