Constantine I, also known as Constantine the Great, was one of the most influential Roman emperors in history. He was born in Naissus, in the province of Moesia (now Niš, Serbia), around 272 AD. His father was Flavius Constantius, a Roman army officer who later became a co-emperor of the Tetrarchy, a system of four rulers established by Diocletian to stabilize the empire. His mother was Helena, a Greek woman of humble origin who later became a Christian and a saint.

Constantine grew up in the eastern part of the empire, where he received a classical education and learned to speak both Latin and Greek. He also served in the military under Diocletian and Galerius, two of the Tetrarchs. In 305 AD, when Diocletian and Maximian abdicated, Constantius became the senior Augustus (emperor) in the west, and Constantine joined him in Britain. The following year, Constantius died at York, and Constantine was proclaimed emperor by his troops.

However, Constantine was not the only claimant to the throne. He had to fight against Maxentius, the son of Maximian, who controlled Italy and Africa; Licinius, another Augustus appointed by Galerius; and Maximinus Daia, a Caesar (junior emperor) who ruled Egypt and Syria. After defeating Maxentius at the Battle of Milvian Bridge in 312 AD, Constantine attributed his victory to the Christian God, whom he claimed had shown him a sign of a cross in the sky with the words “in this sign conquer”. He then issued the Edict of Milan in 313 AD, which granted religious tolerance to all faiths, especially Christianity.

Constantine also allied himself with Licinius, who married his half-sister Constantia. Together they defeated Maximinus Daia in 313 AD and divided the empire between them. However, their alliance soon turned into rivalry, and they fought several wars until Constantine defeated Licinius at the Battle of Chrysopolis in 324 AD and became the sole ruler of the Roman Empire.

Constantine was a visionary leader who initiated many reforms that transformed the empire. He reorganized the administration, the army, and the coinage. He founded a new capital city on the site of Byzantium, which he named Constantinople (now Istanbul) after himself. He patronized arts, sciences, and literature. He also supported the Christian Church and convened the First Council of Nicaea in 325 AD, which defined the doctrine of the Trinity and established a uniform date for Easter.

Constantine died in 337 AD at Nicomedia (now İzmit), after being baptized on his deathbed by Eusebius of Nicomedia, a bishop who had supported Arianism, a heretical view that denied the divinity of Christ. He was buried in a mausoleum at the Church of the Holy Apostles in Constantinople. He was succeeded by his three sons: Constantine II, Constantius II, and Constans I.

Constantine is widely regarded as a saint by Eastern Orthodox Christians and some other churches. He is also known as “the Great” for his achievements and legacy. He is considered to be the first Christian emperor and the founder of Byzantium, which later became the Byzantine Empire and lasted for over a thousand years.