Gregory the Illuminator (Grigor Lusavorich; c. 257 – c. 331) is the patron saint and first official head of the Armenian Apostolic Church. He was a religious leader who converted Armenia from paganism to Christianity in 301.

Gregory was the son of the Armenian Parthian nobles Anak the Parthian and Okohe. His father, Anak, was a Prince said to be related to the Arsacid Kings of Armenia or was from the House of Suren, one of the seven branches of the ruling Arsacid dynasty of Sakastan. Anak was charged with assassinating Khosrov II, one of the kings of the Arsacid dynasty and was put to death. Gregory narrowly escaped execution with the help of Sopia and Yevtagh, his caretakers. He was taken to Caesarea in Cappadocia where Sopia and Yevtagh hoped to raise him. Gregory was given to the Christian Holy Father Phirmilianos (Euthalius) to be educated and was brought up as a devout Christian.

Upon coming of age, Gregory married a woman called Miriam, a devout Christian who was the daughter of a Christian Armenian prince in Cappadocia. From their union, Miriam bore Gregory two children, their sons Vrtanes and Aristaces. Through Vrtanes, Gregory and Miriam would have further descendants and when Gregory died, Aristaces succeeded him. At some point, Miriam and Gregory separated in order that Gregory might take up a monastic life. Gregory left Cappadocia and went to Armenia in the hope of atoning for his father's crime by evangelizing his homeland.

At that time Tiridates III, son of the late King Khosrov II, reigned. Influenced partly by the fact that Gregory was the son of his father's enemy, he ordered Gregory imprisoned for twelve (some sources indicate fourteen) years in a pit on the Ararat Plain under the present day church of Khor Virap located near the historical city Artashat in Armenia. Gregory was eventually called forth from his pit in c. 297 to restore sanity to Tiridates III, who had lost all reason after he was betrayed by Roman emperor Diocletian.

In 301 Gregory baptized Tiridates III along with members of the royal court and upper class as Christians. Tiridates III issued a decree by which he granted Gregory full rights to begin carrying out the conversion of the entire nation to the Christian faith. The same year Armenia became the first to adopt Christianity in the early fourth century

The newly built cathedral, the Mother Church in Etchmiadzin became and remains the spiritual and cultural center of Armenian Christianity and center of the Armenian Apostolic Church. Most Armenians were baptized in the Aratsani (upper Euphrates) and Yeraskh (Arax) rivers.

Many of the pre-Christian (traditional Indo-European) festivals and celebrations such as Tyarndarach (Trndez, associated with fire worship) and Vardavar or Vadarvar associated with water worship, that dated back thousands of years, were preserved and continued in the form of Christian celebrations and chants. In 302, Gregory received consecration as Patriarch of Armenia from Leontius of Caesarea.

In 318, Gregory appointed his second son Aristaces as the next Catholicos in line of Armenia's Holy Apostolic Church to stabilize and continue strengthening Christianity not only in Armenia, but also in the Caucasus. Gregory also placed and instructed his grandson Gregory (one of the sons of Vrtanes) in charge of the holy missions to the peoples and tribes of all of the Caucasus and Caucasian Albania; the younger man was martyred by a fanatical mob while preaching in Albania.

In his later years, Gregory withdrew to a small sanctuary near Mount Sebuh (Mt. Sepuh) in the Daranali province (Manyats Ayr, Upper Armenia) with a small convent of monks, where he remained until his death.

After his death his corpse was removed to the village of Thodanum (T'ordan, modern Doğanköy, near Erzincan). His relics were scattered near and far in the reign of the Eastern Roman Emperor Zeno. In the 8th century, the iconoclast decrees in Greece caused a number of religious orders to flee the Byzantine Empire and seek refuge elsewhere. San Gregorio Armeno in Naples was built in that century over the remains of a Roman temple dedicated to Ceres, by a group of nuns escaping from the Byzantine Empire with the relics of Gregory, including his skull, a femur bone, his staff, the leather straps used in his torture and the manacles that held the saint. The femur and manacles were returned by Pope John Paul II to Catholicos Karekin II and are now enshrined at Saint Gregory the Illuminator Cathedral in Yerevan. The Gregoriou Monastery on Mount Athos also claims to have the skull. The Mother See of Holy Etchmiadzin and the Holy See of Cilicia in Antelias, Lebanon each claim to have the right arm of the saint, in an arm-shaped reliquary, which is used for the blessing the Holy Myron every seven years.

On 20 February, 1743, Nardò, Italy was hit by a devastating earthquake that destroyed almost the entire city. The only structure to survive intact after the quake was the city's statue of St. Gregory the Illuminator. According to the city's registers, only 350 out of the city's 10,000 inhabitants died in the earthquake, leading the inhabitants to believe that St. Gregory saved the city. Every year, they mark the anniversary of the earthquake by holding three days of celebrations in his honor. Relics of the saint are kept at Nardò Cathedral.^[

A number of prayers, and about thirty of the canonical hymns of the Armenian Church, are ascribed to Gregory the Illuminator. Homilies of his appeared for the first time in a work called Haschacnapadum at Constantinople in 1737; a century afterwards a Greek translation was published at Venice by the Mekhiterists; and they have since been edited in German by J M Schmid (Ratisbon, 1872). The original authorities for Gregory's life are Agathangelos, whose History of Tiridates was published by the Mekhitarists in 1835; Moses of Chorene, Historiae Armenicae; and Symeon the Metaphrast.

A Life of Gregory by the Vartabed Matthew was published in the Armenian language at Venice in 1749 and was translated into English by Father Malan (1868).

In the calendar of the Armenian Church, the discovery of the relics of St. Gregory is an important feast and is commemorated on the Saturday before the Fourth Sunday after Pentecost Two other feast days in the Armenian Apostolic Church are devoted to St. Gregory: the feast of his entry into Khor Virap, the 'deep pit or dungeon' (commemorated on the last Saturday of Lent) and his deliverance from Khor Virap (commemorated on the Saturday before the second Sunday after Pentecost). Gregory is commemorated on September 30 by the Eastern Orthodox Church, which styles him "Holy Hieromartyr Gregory, Bishop of Greater Armenia, Equal of the Apostles and Enlightener of Armenia." He is listed on September 30 in the Roman Martyrology of the Ordinary Form of the Catholic Church; his feast day is listed as October 1 in the Extraordinary Form. He is honored with a feast day on the liturgical calendar of the Episcopal Church (USA) on March 23.