The convening of the First Ecumenical Council of 325 AD, held in the city of Nicaea in modern-day Turkey, was one of the most important events in the history of Christianity. It established what came to be known as the Nicene Creed, a definitive statement of belief that has formed the basis of mainstream Christianity ever since.
At the time of Nicaea, the Church was riven with dissent. Different factions within the Church held diametrically opposed theologies, particularly relating to the nature of Christ.
Some Christians, called Arians after the priest Arius, believed that Jesus had been created by God. According to them, God is uniquely self-existent—that is, not created by anything else—but that the same cannot be said of the Son. As the Son was created by God, this meant that there was a time when he didn't exist.
Such a formulation was strongly opposed by other Christians. To them, this implied that Jesus was somehow less than God and therefore not fully divine. As such, they regarded Arianism as nothing less than heresy.
It was largely as an attempt to deal with the increasingly bitter dispute between Arians and their opponents that the Nicene Creed was adopted. The Creed stated that the Son, Jesus Christ, was cosubstantial with the Father, meaning that he was of the same divine substance as God. Furthermore, Jesus was affirmed as "begotten" of the Father, not made, which meant that he was not less than God, as the Arians held.