The second period covers most of the second century and the first half of the third century, and its main content is the process of formation of the Christian Church. At this time, there is a transformation of small and scattered first Christian communities in a fairly clear and coherent organization-the Church, at the head of each of the divisions of which is vested with special powers of the head of the community-the Bishop. There is a separation of Christians into clergy and laity, was started in the territorial structure of the Church, going to the first councils of bishops, yet local, formalized Christian ritual and Liturgy, erected the first Christian churches.

From the process of formation of the Church was inseparable the process of registration of Christian dogma. The first wave of semi-anonymous and anonymous authors of the Apostolic age, of whom we know very little and who laid the foundations of Christian theology, was followed by a wave of Christian theologians, philosophers and apologists, of whom we know more and from whom many writings have come down to us. The second half of the second century and the first half of the third century accounted for the activities of such prominent Christian thinkers as Irenaeus, Tertullian, Clement of Alexandria, Cyprian and Origen. In their works were outlined the main directions of further development of the Christian doctrine, formulated many of the theological principles of Christianity, put forward ideas that later became the Foundation of Christian philosophy. These thinkers prepared the conditions for the flourishing of Christian Patristics in the next century.

Finally, at this time there is a spread of Christianity throughout the Roman Empire and in all social strata of Roman society, although the proportion of Christians among the lower strata remained much greater than among the higher strata.

Important features of this period were the struggle of Christians and the emerging Church with both internal and external opponents. Inside, the main danger was represented by the Gnostics and other heresies that appeared one after another (Docetism, Montanism, marchionism, etc.). The struggle against the emerging heresies proved to be a test of the strength of both the foundations of the emerging Christian creed and the emerging organizational structures of the Church. However, without the support of the state, the fight against internal opponents was not effective enough.

From the outside, the new religion was fiercely attacked by pagan thinkers (Lucian, caecilius, Celsus, etc.), which caused a wave of controversy on the part of Christian writers and led to the emergence of an apologetic direction in Christian literature. Apologists had to respond not only to the arguments of their direct opponents, but also to prove the usefulness of Christianity for the state, as well as to refute the opinions about the evils of Christianity, which existed among ordinary people.

In view of the growing number of Christians and the design of Church structures, Christianity by the beginning of the III century. became quite a real force, which could not ignore the state. The third period – from the middle of the III century to the beginning of the IV century. can be called the era of persecution, as it is the relationship of Church and state acquire at this time of paramount importance for Christianity. The new historical conditions – the General crisis of the Roman Empire-not only set new challenges for Christianity, but the policy of persecution pursued by the Imperial power at this time puts on the agenda the question of the survival of the new religion and the emerging Church. The blows dealt to Christianity during this period were the most severe test of its existence. Christianity not only survived in this cruel era, not only preserved the Church organization and the basic values of the creed, but also grew, strengthened and multiplied the number of its supporters.

The fourth period in the history of Christianity spans almost the entire fourth century, beginning with the edict of Milan by the emperors Constantine and Licinius in 313, when Christianity was granted legal status in the Roman Empire and ending with the reign of Theodosius the Great, who made Christianity the only official religion in the state. This period is called the time of Christianization of the Roman Empire.

Christianization is a process of interaction and mutual influence of all structures of late-antique civilization (economy, social sphere, state, ideology and culture) and Christian religion with all its institutions (Church as a political and economic organism, Christian ideology and culture). In fact, Christianization, as a process of interaction between Christianity and various secular structures in different historical epochs, is the main content of the entire history of Christianity. Nicene (1st Ecumenical) Council of 325 as any historical process Christianization had not only a religious side, but also social, political, economic and cultural sides.

Fifth period – from the fourth to the eighth century was a time of Ecumenical councils, seven of which are accepted as such in Orthodoxy and in Catholicism. These councils were aimed at strengthening the unity of the Church and the fight against heresies, the formation of canonical rules, the definition of holidays and rituals, strengthening the foundations of the doctrine.