Religion in pre-Islamic Arabia was historically traditional, local polytheistic religions. In the fourth century, Christianity became the dominant religion in the Roman Empire, Armenia, and Ethiopia, and began to make ground among large populations of Roman Arabs across the Sinai Peninsula, Jordan, southern Syria, and in the interior of the Arabian Peninsula, where large populations of Arab Christians emerged.
The most significant populations of Arab Christians formed among Roman populations, especially in the province of Roman Arabia, and on the Arabian Peninsula, in Eastern Arabia and South Arabia, with local populations also forming in the Hejaz. With the growth of Christianity in the Arabian Peninsula in the fifth and sixth centuries, churches, monasteries, cathedrals, and martyria were built. These new institutions allowed local leaders to display benefaction towards their communities, communicate with local leaders, and act as points of communication with Christian communities abroad, such as Byzantine Christians. Major Christian centers were assigned into bishoprics, so that a bishop could act as the local religious leader for the Christians in the region. Bishoprics appeared across Eastern Arabia, North Arabia, and South Arabia (especially in Najran and Zafar).
The growth of Christianity on the Arabian Peninsula was driven by proselytism: the mains sources of proselytism were missionaries from Christian churches, especially Syrian churches, from the Byzantine Empire, Christian Iraqi populations in the Sasanian Empire (particularly from Al-Hira), and Ethiopian Christians from the newly converted Kingdom of Aksum, who, in the early sixth century, conquered Himyar, the ruling power over South Arabia. Byzantine Christians wrote many stories about holy men and miracle workers active in converting Arabs, turning them away from pagan and polytheistic idolatry. Arabian Christianity is also known from the Quran, the holy scripture of Islam, and a growing number of pre-Islamic Arabian inscriptions. Arabian Christianity did not immediately decline following the early Muslim conquests, with the most severe economic and other pressures leading to the decline of the Christian communities coming in the eighth century onwards.
Arab Christianity in the Roman Empire Outside of the Arabian Peninsula, large populations of Arabs lived in the Roman Empire and in other places in the Near East. After the conversion of Constantine the Great to Christianity in the early fourth century, large swathes of these Arabs converted to Christianity. Even before this, Abgar VIII, the Arab king of the Osroene Kingdom (known also as the "Kingdom of Edessa") converted to Christianity around 200 AD, making Abgar the first head of state in world history to convert to Christianity, and the Edessan kingdom the first Christian state. Abgar's conversion also provided a place of protection for Christians in the time from Roman persecution, and turned Edessa into a spiritual capital of Syriac Christianity, a counterpart to the role played by Antioch in Greek Christianity. Even before Constantine, the emperor Philip the Arab was also speculated by fourth-century Christians to have privately converted to Christianity, although his reign was short.
Constantine's conversion ushered in a three-century period of the rise of Christianity among Roman Arab populations. In Syria, this included the conversion of the tribe of the Tanukhids in the fourth century, the Salihids in the fifth century, and in the sixth century, the Ghassanids, who rose to power as Rome's primary and most powerful Arab client kingdom at this time. Their doctrine was Monophysitism, and they widely sponsored Christian churches and monasteries in their areas across Syria. In Mesopotamia (Iraq), this was initiated by Abgar's conversion; his territory was conquered by the Romans in the 240s AD. After this conquest, the Mesopotamian center of the region transitioned from Edessa to Al-Hira, the capital of the Lakhmid kingdom. The Lakhmids ruled Al-Hira for three centuries, during which it became a major Christian center. The city became an episcopal see with a bishop around 410 AD. Vast numbers of churches and convents were built, to the point that it fascinated the eighth-century Muslim author Hisham ibn al-Kalbi enough to write a book about it, which has largely been lost, except for quotation of a long inscription he documented from later Islamic authors. Christian Hira may have even been the place where the Arabic script was developed. Towards the very end of the Lakhmid period, the final Lakhmid king, Al-Nu'man III, also converted to Christianity.
North Arabia
The first accounts of Christians and Christian conversion in Arabia appear in the New Testament in the first century, including in Paul the Apostle's Letter to the Galatians (1:15–17) and the Book of Acts (Acts 2:8, 11). In the mid-third century, Christian institutions organized two church councils, known now as the Councils of Arabia, to condemn the heresy of Beryllus of Bostra, the earliest recorded bishop in the province of Roman Arabia seated in the metropolis Bostra, for believing that both body and soul are lost between death and the Last Judgement (Ecclesiastical History, 6:37). By the fourth century, significant communities of Christians existed among desert Arabs, especially in the Sinai Peninsula and the province of Roman Arabia. Saint Moses, named "Apostle of the Saracens" by the Romans, was a hermit and, at the behest of the Syrian Arab warrior-queen Mawiyya of the Tanukhid tribe, became another one of the first known Arab bishops.
The growth of Christianity among Arabs was fuelled by missionary efforts. Many hagiographical sources record stories of Christian holy men converting groups of Arabs to Christianity. These stories followed a common template: first, an Arabian community interacts with a monk (or other kind of holy man). Shortly afterwards, the community renounces polytheism and idol worship. Finally, a church is built. Missionaries described in these accounts included Ahudemmeh (d. 575), Euthymius the Great (d. 473), Simeon Stylites (died 459), and the events leading to the construction of the shrine of St. Sergius at Resafa patronized by Al-Mundhir III, leader of the Ghassanid tribe.
Jerome (in the Life of St Hilarion 16.1–12) says that Saint Hilarion converted the Arabs of Elusa, a city located southwest of the Dead Sea, who worshipped the goddess Venus. After exorcising demons from many of their members, they flocked to him and ask for his blessing. Idol worship ended and Hilarion helped them lay the plan for building a church before his departure. The Life of Euthymius by Cyril of Scythopolis says that Euthymius the Great, the abbot of Israel, was approached by an Arab of the Persian army, Aspebetos, to cure his sons sickness. When Euthymius did this, Aspebetos converted and defected to the Romans along with the rest of his clan. Another figure, Ahudemmeh, was said to "visit all the camps of the Arabs, instructing and teaching them in many sermons .... establishing in every tribe a priest and a deacon ... and founding churches and naming them after tribal chiefs."
A number of known tribal conversions to Christianity happened from the fourth to sixth centuries, including of the then-dominant Byzantine foederati, the Salihids, around 400 (following the decision of their leader Zokomos), the Ghassanids at the beginning of the reign of their leader Al-Harith ibn Jabalah (r. 528–569), and the Lakhmids during the reign of their final king of the Nasrid dynasty, Al-Nu'man III, in the late 6th century. There is also some evidence that the Taghlib and Tanūkhid tribes had converted.
Arab kings became involved in imperial church affairs and in theological debates, and helped organize meetings between members at the highest authorities in the churches of the empire. The most important of these were the Ghassanids, who controlled a kingdom in the Levant and in northern Arabia. John of Ephesus describes the convert king, al-Harith, as helping appoint bishops and exercising authority in the "southern and [eastern] countries and in the whole of the desert and in Arabia [Petraea] and Palestine". The Ghassanids became leading patrons of the Miaphysites and sponsored the martyr cult of St Sergius, popular among Arabs. Mutually, the Miaphysites sent missionaries into Arabia. Epigraphic evidence suggests they sponsored multiple Christian sites, including the shrine of St Sergius, a basilica in al-Ruṣāfa, and a three-church complex in Nitl (near Madaba). The Ghassanids are also linked to three Christian Paleo-Arabic inscriptions from Syria, including the Jebel Usays inscription, the Harran inscription, and the Zabad inscription. The Zabad inscription was found at a martyrium at the Church of St. Sergius. Other inscriptions at martyria mention more sponsors with Arab names.
Around the year 569 or 570, Al-Harith ibn Jabalah convened a conference in the province of Roman Arabia towards resolving a theological debate at the time. A letter from the conference has survived, known as the Letter of the Archimandrites (preserved in the Documenta Monophysitarum), and it contains the signatures of the representatives (being the Archimandrite) of 137 different monasteries, indicating a widespread systematic presence of Christians in Roman Arabia, territorially centered in modern-day Jordan and adjacent regions.
A growing amount of archaeological evidence for Christians in the region is known. A monastery has been discovered at Kilwa, located in northwest Arabia in the Tabuk Province of Saudi Arabia, dedicated to Saint Thecla, a disciple of Paul the Apostle. The monastery might have been established at the eve of Islam. Its presence in the arid desert indicates that some nomadic Arab tribes had converted by that time. A growing number of Christian inscriptions in the area are also known, especially in the Paleo-Arabic script, such as the Jebel Usays inscription, Harran inscription, Zabad inscription, and the Umm al-Jimal inscription. The Umm al-Jimal inscription was discovered in the northern part of the Double Church at Umm al-Jimal. In 2021, the first Christian Safaitic inscription, dating to the fourth century, was found, invoking the aid of Jesus to cure the maternal uncle of the author. The text calls Jesus ʿĪsâ, the earliest use of this grammatical form before the Quran.
South Arabia Christianity was introduced into South Arabia in the fourth century. Towards the end of the fifth century, Christianity's presence was strong enough that the Synodicon orientale says that a "Moses of Himyar" attended a synod in 486 AD. During the sixth century, bishoprics are described in the capital Zafar (Gregentios), Najran, and Qana'. It was also in this period that the Christian community of Najran faced severe persecution, precipitating an invasion by the nearby Christian Ethiopian Kingdom of Aksum, leading to official Christian rule for decades.
Early missionary efforts According to the Greek historian Philostorgius (d. 439) in his Ecclesiastical History 3.4, Constantius II, the successor of Constantine the Great, sent an Arian bishop known as Theophilus the Indian (also known as "Theophilus of Yemen") to Tharan Yuhanim, then the king of the South Arabian Himyarite Kingdom to convert the people to Christianity. According to the report, Theophilus succeeded in establishing three churches, one of them in the capital Zafar. From the fifth and sixth centuries, the Miaphysite church displayed a significant interest in expanding missionary activity in the Himyarite Kingdom.