Originally, Satan was rash enough to believe that he could directly compete with Yahweh. He had won a third of the angels (Revelation 12:4), he had won Adam and Eve (Genesis 3:1-7), taking Adam's place as prince of this world (John 12:31), and he thought he could win mankind.

Even then, Satan appeared to Eve disguised as a serpent (Genesis 3:1). Men quickly degenerated and Satan appeared largely unmasked.

Satanism is the direct worship of Satan. With time, Satan invented many other false gods. Worship of Satan through the names of false gods is paganism. Either way (Satanism or Paganism), if the True God isn't being worshiped, Satan is.

In this manifestation, Satan didn't try to hide his involvement. He knew he was being worshiped, most of the worshipers knew it, and Satan was out in the open, happy to have the worship that was due Yahweh.

This operation was largely overcome by Jesus, who won a great victory over Satan. Jesus, as the second Adam, won back the title of prince of this world. This was the defeat of Satan (John 12:31). The war was over, but battles continue.

Satan regrouped and went "underground".