Photinus (Greek Φωτεινός; died 376), was a Christian bishop of Sirmium in Pannonia Secunda (today the town Sremska Mitrovica in Serbia), best known for denying the incarnation of Christ, thus being considered a heresiarch by the Catholic Church. His name became synonymous in later literature for someone asserting that Christ was not God. His teachings are mentioned by various ancient authors, like Ambrosiaster (Pseudo-Ambrose), Hilary of Poitiers, Socrates Scholasticus, Sozomen, Ambrose of Milan, Augustine of Hippo, John Cassian, Sulpicius Severus, Jerome, Vigilius of Thapsus and many others.
Photinus grew up in Ancyra in Galatia, where he was a student and later a deacon of bishop Marcellus. Marcellus, in later life a staunch opponent of Arianism, was excommunicated and deposed in 336 but rehabilitated by the Synod of Serdica in 343, which also made Photinus bishop of Sirmium. In 344, the Synod of Antioch deposed Marcellus and drew up the Macrostich, a creed which listed their beliefs and objections to Marcellus's doctrines (among others). R. P. C. Hanson (1973) described Photinus' Christology as consistent with the early teachings of Marcellus. between 340 and 350.
At the time Photinus voiced his own theological system, according to which Jesus was not divine and the Logos did not exist before the conception of Jesus. For Photinus the Logos was simply a mode of manifestation of the Father, hence he denied the pre-existence of Christ and saw theophanies in the Old Testament as of the father, and the image of the "Son of God" (actually, Son of man) in front of (and distinct from) the Ancient of Days as prediction only. Photinus's apprehension of God as Father, and his teachings about the nature of Jesus Christ are maybe more complex than has been thought.
The church historian Socrates Scholasticus identifies Photinus' beliefs with those of Sabellius, Paul of Samosata and Marcellus. This also was presumably misapprehension of Photinus' doctrine about Jesus. Ambrose, among the many accusing Photinus of reducing Christ to a man adopted by God, notes that his favourite verses were 1 Timothy 2:5 and John 8:40. In the controversies against Polish Socinian influence in 18th-Century Photinus was repeatedly cast as a heretical predecessor of early Unitarians for his denial of the pre-existence of Christ.
Synods held in 345 and 347 excommunicated Photinus, but Photinus remained in office, due to his popular support. A synod at Sirmium was held and Hilarius of Poitiers quotes some of its Arian propositions.
Photinus appealed to emperor Constantius II. At another synod in Sirmium in 351, Photinus argued with the semi-Arian Basil of Ancyra and Photinus was deposed on charges of Sabellianism and Adoptionism. He was anathematised and sent into exile, where he wrote several theological works.
He returned to his see during the reign of Emperor Julian, who wroted him an approving letter from Julian the Apostate in AD 362, which attacked Diodore of Tarsus, then engaged in combatting Julian's attempts to de-Christianize the empire, and began:
- O Photinus, you at any rate seem to maintain what is probably true, and come nearest to being saved, and do well to believe that he whom one holds to be a god can by no means be brought into the womb. But Diodorus, a charlatan priest of the Nazarene, when he tries to give point to that nonsensical theory about the womb by artifices and juggler's tricks, is clearly a sharp-witted sophist of that creed of the country-folk.
Ambrosiaster, in the next generation, probably referred to this letter when he commented that Photinus 'because he did not regard Christ as God on the grounds that he was born, he appears wise to the worldly.'
According to Jerome, Emperor Valentinian I (364-375) exiled Photinus again. In about 365, a letter from Liberius, bishop of Rome, to several Macedonian bishops mentions a bishop called Photinus among the latter. It is unlikely that this refers to Photinus, bishop of Sirmium, whose see was located in Pannonia) and not Macedonia.
After being exiled by Valentinian, Photinus settled in his native Galatia and his doctrines, Photinianism, died in the West. By the time of Augustine, a "Photinian" was anyone who believed Christ was a mere man.
- About annum of his death and previous attempts to identify him with some other bishops with the same name, from the territory of Illyricum, before and after the year 376., see work that contradicts this earlier views: Aleksandar Ivanović, Fotin Sirmijumski i njegov nauk- novi pokušaj rekonstrukcije fotinističke Hristologije (Photinus of Sirmium and his Doctrine- New Attempt of Reconstruction of Photinian Christology, p. 4. 
- Aleksandar Ivanović, Fotin Sirmijumski i njegov nauk- novi pokušaj rekonstrukcije fotinističke Hristologije (Photinus of Sirmium and his Doctrine- New Attempt of Reconstruction of Photinian Christology, p. 1. .
- Susanna Elm, Sons of Hellenism, Fathers of the Church 0520951654 2012 p233 "What did Photinus argue, and where was he in the early 360s? None of his writings are extant, and we know little of his whereabouts; much has to be reconstructed from what opponents such as Gregory say."
- Kari Kloos Christ, Creation, and the Vision of God 2010 Page 75 "On the difficulty of identifying the doctrine of Photinus, see Charles Hefele, A History of the Councils of the Church from the Original Documents, vol. 2 (Edinburgh: T&T Clark, 1876), 186–193. Due to the lack of extant writings by Photinus and ..."
- The New Westminster Dictionary of Church History: Robert Benedetto, James O. Duke - 2008 "As none of Photinus's writings are extant, evidence for his views comes from his detractors. His theology was condemned by at least two small synods, in Milan and Rome"
- Socrates Scholasticus, Church History, book 2, chapter 18.
- R. P. C. Hanson (1916-1988), The Search for the Christian Doctrine of God: The Arian Controversy, 318-381 (9780801031465): 1973 "Photinus' doctrine appears to have been a form of what might be called middle Marcellism, i.e. what Mercellus[clarification needed] originally taught before his vicissitudes caused him to temper the edge of his doctrine and take account of the criticisms of his friends as well as of his enemies, a little more moderated. He certainly taught that the human body of Jesus had a human mind or soul, insisting on his wholeness, and we have seen that this was entirely consistent with the teaching of Marcellus though in his early period of theological writing at least Marcellus does not seem to have held it."
- Hanson "There does not seem to be anything very original about Photinus' teaching. He was a doctrinaire disciple of Marcellus. He does not reflect Marcellus' idiosyncratic concept of the limited term for Christ's kingdom, and is more emphatic than Marcellus about the human soul of Christ. He probably reflects the position of Marcellus between 340 and 350, and perhaps might cause us to consider again the conjecture discussed above that Marvellus did in his middle or later period admit a human soul to Christ, and that he played down his more eccentric ideas. Photinus must have represented a considerable embarrassment not only to Marcellus, whom he followed all too faithfully, but also to all Western theologians. He displayed to them what could happen to those who insisted rigorously that there was only one hypostasis in the Godhead and how near they were to falling over the precipice of Sabellianism. He may also have contributed to a prejudice against, or a blindness to the necessity of, ascribing a human soul to Jesus."
- Sozomen, Church History, book 4, chapter 6.
- Hanson: "Photinus appealed particularly to Isa 44:6 ('I am God and there is no other') in order to articulate a determinedly monothestic[clarification needed] doctrine of God. The Logos for him was simply a mode of manifestation of the Father, a power or aspect of him not in any serious sense distinct from him. He apparently coined the word 'Wordfather' and regarded the Logos as wholly interchangeable with 'God'. Like Marcellus, he favoured the analogy of a man and his thought for the relationship of the Father to the Son. He could hardly have used the concept of begetting in the circumstances. Like the early Marcellus, he distinguished sharply the Son from the Father; the Son did not come into existence until the Incarnation and was defined as the whole human being who was born of Mary; Christ had no pre-existence; theophanies in the OT were only God appearing. When the OT spoke of a Son of God (as at Dan 7:13) this was prediction only.
- Aleksandar Ivanović, Fotin Sirmijumski i njegov nauk- novi pokušaj rekonstrukcije fotinističke Hristologije (Photinus of Sirmium and his Doctrine- New Attempt of Reconstruction of Photinian Christology, p. 2-3. .
- Socrates Scholasticus, Church History, book 2, chapters 18 & 19.
- Socrates Scholasticus, Church History, book 2, chapter 29.
- Aleksandar Ivanović, Fotin Sirmijumski i njegov nauk- novi pokušaj rekonstrukcije fotinističke Hristologije (Photinus of Sirmium and his Doctrine- New Attempt of Reconstruction of Photinian Christology, p. 2. 
- Hanson "Everybody in the ancient world accuses Photinus of reducing Christ to a mere man adopted by God, i.e. the union between Logos and man was one of inspiration and moral agreement only. Ambrose tells us that two favourite texts of Phontinus were I Tim 2:5 ('there is one God and one mediator between God and man, the man Jesus Christ'), and John 8:40 ('You seek to kill me, a man who has spoken the truth to you'), and one can see why. There is evidenced here a consistent determination to avoid recognising any distinctions in the being of God."
- Edward Harwood Of the Socinian scheme. 1786 Second edition, enlarged - Page 16 "Photinus, the disciple of Marcellus, inherited from his master the same religious fystem. He asserted, that there was one supreme Being, who had created all things by his own word, but he denied the eternal generation and pre-existence of the son, and maintained that Christ had no being before he was born of his mother.
- Julian the Apostate, Letters, Loeb Classical Library volume 3, p. 186-7
- Hanson: "A recent study of references to Photinus in Ambrosiaster (fl in Rome 363-384) by Lydia A. Speller confirms all these points. Ambrosiaster says of Photinus, 'because he did not regard Christ as God on the grounds that he was born, he appears wise to the worldly.' This is probably a reference to the fact that the Emperor Julian, in a letter, part of which is extant, addressed to Photinus (c. 362), congratulated Photinus on his refusal to believe that a god could be introduced into the womb."
- Socrates Scholasticus, Church History, book 4, chapter 12.
- Aleksandar Ivanović, Fotin Sirmijumski i njegov nauk- novi pokušaj rekonstrukcije fotinističke Hristologije (Photinus of Sirmium and his Doctrine- New Attempt of Reconstruction of Photinian Christology, p. 3-4.