The Donatists, who establish their own communities, appoint bishops, and convene church councils, vigorously resist opposition from both the traditional church and the Roman state.

The rigoristic Donatist teachings proclaim that only the sinless can belong to the true church, that sacraments conferred by sinful ministers are invalid, and that only baptism conferred by a Donatist is valid.

Because the Donatists had denied the representative character of two earlier synods, at Rome and in Africa, at which they had been condemned, Constantine convenes the first representative meeting of Christian bishops in the Western Roman Empire at Arles in southern Gaul in August 314.

Attended by representatives of forty-three bishoprics, the Donatists are again condemned, but they reject the decisions reached by the council and again appeal to Constantine to review their case.

Licinius and Constantine now control both halves of the empire.

When Licinius attempts in 314 to foment a revolt against Constantine, the latter responds by leading twenty thousand men into the eastern empire.

After fighting the inconclusive Battle of Cibalae in southern Pannonia on October 8, 314, the two rulers conclude a truce.

The Roman Church proselytizes and derogates Jews, but tolerates them so that they may witness the return of the Messiah, i. e. Jesus.

Constantine begins issuing edicts against the Jews, denouncing them as Christ killers.

The Code of Constantine, issued in 315, limits the rights of non-Christians.

The Arch of Constantine, commissioned by the Senate in Rome to mark his victory at the Milvian Bridge and constructed near the Coliseum in around 315, bears an inscription that attributes Constantine's success to the "prompting of a deity."

The deity is unnamed by the Senate, who probably picture a pagan god, but Rome’s Christians view this deity as their own god.

The Arch’s relief decoration, carved in Carrara marble, bears stylistic correspondences to Christian sarcophagi of the age.

The provincial and folk art roots of the sculptural reliefs indicate a new trend in artistic taste, a reflection of the humble origins of most of Rome’s Christians.

Constantine's Victory Over Licinius and the Unification of the Roman Empire (324 CE)

In 324 CE, Constantine the Great decisively defeated his brother-in-law and co-emperor, Licinius, bringing an end to their power struggle and reuniting the Roman Empire under a single ruler. This marked the final collapse of the Tetrarchy, firmly establishing Constantine as sole emperor, a position he would hold until his death in 337 CE.

1. Background: The Tensions Between Constantine and Licinius

• After the defeat of Maxentius at the Battle of the Milvian Bridge (312 CE), Constantine controlled the Western Roman Empire.
• Meanwhile, Licinius ruled the Eastern Roman Empire, having defeated Maximinus Daia in 313 CE.
• Although they had issued the Edict of Milan (313 CE) together, tensions grew due to:
  • Constantine’s increasing favoritism toward Christianity, while Licinius remained suspicious of the new faith.
  • Licinius’ persecution of Christians, likely to counteract Constantine’s growing influence in the East.
  • Political and military rivalry between their courts, leading to open conflict.

2. The Civil War and Constantine’s Campaign (324 CE)

The final war between Constantine and Licinius was fought in two major battles:

1. The Battle of Adrianople (July 3, 324 CE)

   • Constantine’s army invaded Thrace, confronting Licinius near Adrianople.
   • Despite having a numerically superior army, Licinius suffered a devastating defeat, forcing him to retreat across the Bosphorus to Asia Minor.

2. The Battle of Chrysopolis (September 18, 324 CE)

   • Constantine pursued Licinius, engaging him in Bithynia (modern-day Turkey).
   • This final battle was a decisive victory for Constantine, leaving Licinius with no escape.
   • Licinius surrendered, bringing an end to the division of the empire.

3. Aftermath: Constantine Becomes Sole Ruler of Rome

• Licinius was initially spared but later executed in 325 CE, likely due to concerns over a potential rebellion.
• With both the Western and Eastern Roman Empires reunited, Constantine became the undisputed ruler, the first sole emperor since Diocletian’s reforms in 286 CE.

4. Significance of Constantine’s Victory

• End of the Tetrarchy: The system of divided rule was permanently dismantled, and the empire returned to a single ruler model.
• Christianity’s Growing Dominance: With Constantine’s victory, Christianity became increasingly favored, paving the way for its eventual establishment as the state religion.
• Foundation of Constantinople (330 CE): Constantine’s new capital in the East ensured the empire’s long-term survival, eventually evolving into the Byzantine Empire.

Conclusion: A Turning Point in Roman History

Constantine’s defeat of Licinius in 324 CE was a pivotal moment that not only ended the civil wars of the Tetrarchybut also set the stage for the transformation of the Roman Empire into a Christian empire. His rule reshaped the course of Western civilization, leading to an era of imperial stability and religious revolution.

Roman emperor Constantine issues an edict in favor of slaves in 316, prohibiting their punishment by crucifixion and facial branding.

Constantine prevails again against his Eastern counterpart Licinius (shortly after he had named as co-emperor Valerius Valens, who had previously been dux limitis in Dacia) in the plain of Mardia (also known as Campus Ardiensis) in Thrace.

The emperors are reconciled after this battle.

Licinius according to a peace signed on March 1, 317, has to execute Valerius Valens, and cedes territory to Constantine, including …

…the provinces of Pannonia and Moesia.

Constantine had summoned to Trier the Christian theologian and polemicist Lactantius, to be the tutor of Crispus, who on March 1, 317, is given he title of caesar and made titular ruler of Gaul; Constantine's second son receives the title of Caesar at the same time.

The Diocese of Gaul (Latin: Dioecesis Galliarum, "diocese of the Gaul [province]s"), established in about 314, encompasses northern and eastern Gaul; that is, modern France north and east of the Loire, including the Low Countries and modern Germany west of the Rhine.

The diocese comprises the following provinces: Gallia Lugdunensis I, Gallia Lugdunensis II, Gallia Lugdunensis III, Gallia Lugdunensis IV (Senonia), Belgica I, Belgica II, Germania I, Germania II, Alpes Poenninae et Graiae and Maxima Sequanorum.

Constantine in 319 prohibits the separation of the families of slaves during a change in ownership.

Licinius has built up his army and accumulated a huge reserve of treasure during the years of uneasy peace between the Roman Empire’s two Augusti.

Having eventually become alienated from the Christians, he initiates a mild form of persecution in about 320, prompting contemporaries to paint the growing conflict between Constantine and his eastern counterpart as a battle between the forces of paganism and Christianity.