By 324, Constantine and Licinius had built up such a rivalry that it resulted in war. Constantine defeated and captured Licinius and became sole ruler of Rome. After that, Constantine made Constantinople (in Turkey) the capital and center of the Roman government.

Even though Christianity was spreading throughout the empire, it had problems with different groups who were expressing various beliefs. In 325, Constantine presided over the first general ecumenical (world-wide) council of the Christian Church. The council met in Nicaea (northwest Turkey) to settle these disputes among the Christians.

The major issue to settle was a heresy known as Arianism. This belief stated that Jesus Christ was of a different substance than God. The Council of Nicaea condemned Arianism and made up a statement of basic beliefs called the Nicene Creed. This statement said that Jesus Christ was of the same substance as God. Arianism said there are some aspects of Jesus that were similar to God, but not identical. The Nicene Creed was supposed to resolve these questions about the Trinity (three persons in one God) and the divinity of Christ. It added several articles condemning Arianism. A later creed that is also known as the Nicene Creed is more commonly called the Niceno-Constantinopolitan or Constantinopolitan Creed. It is based on a creed made in the Fourth Century by of the Bishop of Jerusalem, who was Cyril (and later Saint Cyril).
 

Although Constantine understood the beliefs of the Christians, he did not become a Christian until later in life. In fact, he did not follow the beliefs, especially when he murdered members of his own family and was ruthless at times in his governing. He taxed the people heavily to pay for his wars and new monuments.