For background history, you can also add The History of the Decline and Fall of the Roman Empire by Edward Gibbon. This is much less concerned with church or Christian history, but it is a heavily sourced work giving the details of the Roman Empire from the early first millenium. Since that's the world Christians lived in, it's important to know what was going on.
Let us not forget: a great many early Christians lived entirely outside the Roman Empire. Indeed, in the 8th century AD, the Church of the East, which existed entirely outside the borders of the Roman Empire, was the largest in the world in terms of geographic territory, and still survives to this day, reduced by an Islamic genocide and probable genocidal acts by Buddhists in Tibet and China to the Fertile Crescent and India, but still extant.
The Holy Apostle Thomas spread the faith along the overland trade route to Kerala, India, which had a Jewish population, the Kochin Jews, since 200 AD, and preached to both Jews and gentiles in Aramaic, leading to his martyrdom when an enraged Hindu maharaja threw a spear at him in 53 AD. The churches he established with his fellow Apostles Addai (Thaddaeus) and Mari (of the seventy) in Edessa, Nineveh and Seleucia-Cstesiphon were all outside the Roman Empire, and grew together with the church in Kerala into the Syriac Orthodox Church and the Church of the East, which at its largest extent strectched from Socotra, an island off the South Coast of Yemen as its southwest extreme, to Persia, in the northwest, and from Persia to Mongolia in the northeast across Mesopotamia, and down through China to Tibet in the southeast.
However, the Mongol-Turkic Islamic despot Tamerlane, also known as Timur the Lame, the national hero of Uzbekistan, where he remains enshrined, started a genocide, continued by his sons, in the twelfth century, that killed all the Christians in those lands outside of the Malabar Coast of India and the Fertile Crescent. However, Kerala is home to the oldest continually operating church, the only first century church still in use, although it was defaced with a Baroque facade when the Portuguese conquered the area and attempted unsuccessfully to impose Catholicism (much of the population remained Orthodox).
Additionally, the first nations to convert to Christianity as their national religion were the Kingdom of Edessa, the aforementioned city state in 301 AD, the Kingdom of Armenia, evangelized by St. Gregory the Illuminator, in 306 AD, and the Empire of Ethiopia and the largest tribe of the Georgian people, who annexed the rest, around 315 AD. While in 314 AD the saintly Emperor Constantine legalized Christianity having himself converted (his mother, St. Helena, was also a Christian largely responsible for the restoration of Jerusalem, so the major pilgrimage sites that exist do so thanks to her leading what could be regarded as the first archeological expedition in history, in the early 320s, Paganism remained legal and in some respects official until the saintly Emperor Theodosius I officially made Christianity the state religion of the Roman Empire and smashed the altar of the goddess Victory in the Curia of the Roman Senate, and closed Pagan temples throughout the Empire, which resulted in Greco-Roman, Egyptian and West Semitic Paganism ceasing to exist in the following centuries. Between the death of Constantine and the coronation of Theodosius, the Roman Empire was not ruled by Christians, but by a series of Arians (practitioners of the heresy that denies the Incarnation and the Trinity, in modern times their error has been revived from obscurity by the Jehovah’s Witnesses, many of the original Millerite Adventists, and a few other groups; it differs from Unitarianism in that it recognizes Jesus Christ as being the Son of God but denies His status as God incarnate and the doctrine of the Trinity). This period of official Arian apostasy was the result of the Arian bishop Eusebius of Nicomedia avoiding directly supporting Arius at the Council of Nicaea, and avoiding attending the First Ecumenical Synod altogether, but instead, after it was done, whispering in the ear of and converting the Emperor’s son and heir Constantius, and to some extent even Constantine himself. The Arian emperors persecuted the Christians, in several cases with violence, for example, in Alexandria, where St. Athanasius, the bishop and successor to St. Alexander of Alexandria, a confessor tortured under Diocletian (his predecessor St. Pepter of Alexandria was martyred), who had anathematized Arius for preaching against the Incarnation, and at the council of Nicaea, St. Athanasius, as protodeacon of the Church of Alexandria, prosecuted Arius. It was at this council where St. Eusebius of Caesarea famously equivocated, but he is recorded by the Syriac Orthodox Church of having later repented of this, which is good to know; most churches do not venerate him because of his equivocation, but since he did repent, and did write such an important work of ecclesiastical history
The existence of substantial numbers of Christians outside the borders of the Roman Empire threatens the traditional Roman vs. Protestant dichotomy even more than the existence of the Eastern Orthodox Church, since some people try to dismiss the Eastern Orthodox as having been subordinate to the Pope even though this is untrue; canons 6 and 7 of the Council of Nicaea declare the churches of Alexandria, Antioch and Jerusalem to have the same authority as the Church of Rome, and canons at the subsequent two councils granted the same privileges to Constantinople, while also recognizing that the Church of Cyprus had also always been ecclesiastically independent. What is more, some Orthodox churches existed in areas which were never a part of the Roman Empire, most notably the Georgian Orthodox Church, which was either the third or fourth national church to be founded, evangelized by Armenia, which was the second country to make Christianity its official religion. Armenia and Georgia were later separated by the EO-OO schism, but this schism is in many places weak and in some places, like Syria, the Antiochian Orthodox (EO) and the Syriac Orthodox (OO) Patriarchates of Antioch have such a close relationship that members of one can receive communion in churches of the other, and neither church will convert members of the other. Unfortunately the relations between the Armenian and Georgian churches have not recovered to the same extent, but both churches share a common architectural style with distinctive conical domes.
Likewise, the Oriental Orthodox existed largely outside the Roman Empire (this was the case with the Armenian Apostolic Church, the Ethiopian Tewahedo Orthodox Church, part of which is now the Eritrean Orthodox Church since the Eritrean independence from Ethiopia in 1994, and roughly half of the Syriac Orthodox Church of Antioch, the portion located in Edessa, Mesopotamia and Persia, which overlaps with the western extent of the Church of the East (sometimes incorrectly called the “Nestorian Church”, which nowadays exists in two churches, the Ancient Church of the East and the Assyrian Church of the East, following a schism in the late 1960s which is in the process of being rectified).
Additionally, the Numidian Orthodox Church was also Oriental Orthodox, but it was killed off by Muslims during the Islamic conquest of the Sudan, Chad and North Africa, which killed all African Christians from antiquity outside of what is now Ethiopia and Eritrea, and Egypt, where the Coptic Orthodox Church, the Greek Orthodox Church of Alexandria, and the Church of Sinai (a small Greek Orthodox jurisdiction under the Greek Patriarch of Jerusalem) coexist, the former having over 10 million members and being the second largest Oriental Orthodox church after the Ethiopian church, which has around 40 million members, and the latter two having just under 100,000 members mainly in Alexandria, who are Alexandrian Greeks by ethnicity, and close ecumenical relations with the Copts, and the Church of Sinai consisting of the Monastery of St. Catharine and a few nearby chapels, which provide medical services to the local Bedouin tribes, some Bedouins being Christian in both Sinai and Jordan (all under the Patriarch of Jerusalem).
