The article explores the question of how the unity of the Roman Empire was construed in the 4th - 5th centuries and what mechanisms ensured its sustainability. By tracing the historical changes of succession, it is evident that Imperial Unity during this period was ensured primarily by Christian ideology and mechanisms for its implementation were the same as those adopted under Diocletian. Although considered as a single realm, the Empire, for the most part of the 4th century was under the control of several emperors, who, if not connected by kinship, would seek to secure their union through dynastic marriages. As power was not inherited by women, the man who married a woman of the royal family, could most likely inherit the throne, but for legitimacy reasons it was necessary for him to be approved by the senior ruler (hence the ordination of Valentinian III by Theodosius II). The year 395, traditionally regarded as the year of the Division of the Empire into Eastern and Western parts, did not bring anything new: the concept of unity and the order of succession remained the same, occasionally disturbed by the actions of usurpers. Rulers can be considered even those who came to power, albeit with the aid of a dynastic marriage, but without the approval of the senior ruler (for example, Marcian, married to Pulcheria, but not approved by Valentinian III). In such cases, emperors sought to legitimize their power by relying on the Church which subsequently allowed to avoid the unpleasantness of being branded a usurper. Ignoring the actual usurpations led to the concept of the divided Empire. Meanwhile, despite the ongoing alienation of the West and the East, the idea of unity of Christendom was not disturbed by such cases; the status of a unified empire was seen as desirable, so the senior Emperor (usually of the East which in that era was more stable) could appoint emperors in the West (as he so did, for example, Leo I). The concept of continuing unity also guided Justinian in his attempt toward restoration.