The Antichrist

The Antichrist is a term commonly used to describe a being who opposes Jesus Christ.

History
Most humans believe that a half-human child conceived by Lucifer is the Antichrist.

According to Castiel, the Antichrist is actually a half-demon/half-human child named Jesse Turner. This child was destined to act as Lucifer's most powerful weapon in Lucifer's war against Heaven. Like Jesus, Jesse was born to a virgin mother, Julia Wright.

Sam Winchester is often referred to as the Antichrist due to the powers he possessed as a result of being a Special Child. Gordon Walker in particular referred to Sam as such.

The son of Lucifer, Jack, has ironically not been referred to as The Antichrist, but more specifically as the Devil's Spawn.

Trivia

 * The term "Antichrist" originated from the Apostle John. It is also found in the New Testament five times, once in plural form. According to John, "Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son."