This article is about a type of people. You may be looking for the Beast out of the Sea, called the "Antichrist".
Antichrist is a designation for anyone who stands against the person and work of Jesus the Messiah. This can be overt rejection by those who have studied His life and claims, or an attack from the outside that does not acknowledge the truth of the Biblical record.
The churches under the Apostle John feared the predicted "man of sin" predicted by Paul might be among the false teachers that were arising in their day. John did not deny this was possible, but warned that even teachers of their day were against Christ. It was necessary to check all claims against what had been taught before and to check the feel and sense of the truth when teachers came talking about spiritual things.
John would later be shown a vision of Satan and his agents coming to power over all the affairs of mankind at the end of the age, including the Beast out of the Sea. This vision certainly confirmed that given to Daniel, and the teachings of Jesus and the Apostle Paul.
The term antichrist comes from the Greek word ἀντίχριστος (transliterated:"antichristos"), which means "opponent of the Messiah". Ἀντίχριστος comes from the two Greek words ἀντί (transliterated: anti) and Χριστός (transliterated: Christos). Ἀντί means "opposite". Χριστός means "anointed or Messiah (Christ)".