This article aims to provide the comprehensive and systematic review of the doctrine of Photinus of Sirmium († 376), based on the work of Hilary of Poitiers De Trinitate composed between 358 and 360. Photinus error is primarily Christological. The first part of the article deals with Hilary’s interpretation of Photinus’understanding of the subject of the Incarnation according to which God the Word/the Word of God was comprehended as a part or one of God’s powers, a mere word, the expression of thought, which does not really differ from God, having no subsistence or existence, so that God is ultimately considered solitary. It is a strict Monarchianism. The second part focuses on Photinus’understanding (based on De Trinita-te) of what was “assumed” of the humanity by the Word of God for the pur-pose of Incarnation, and in which way. Two interpretations referring to Pho-tinus’understanding of the conception of Jesus Christ in Mary, attribute it supernatural causes (the Virginal conception by the non-subsistent Word) and presumably quite natural causes. For the purpose of the Incarnation, the Word of God “assumes” (“takes on”) the entire man, conceived in Mary. The “Incarnation”, as such, is accomplished by the extension of the non-subsisting Word and its in/ dwelling in that man. Based on De Trinitate, the third part deals with the effects of “the Incarnation” as it was understood by Photinus. Hilary concludes that it results in two subjects: on the one hand, it is solus communis generis homo who was born of Mary, and on the other hand, the non-subsistent Word of God that dwelt in that man. The union of the man born of Mary and the Word of God – a part of God’s powers – is reduced, by Photinus and in Hilary’s interpretation, to habitatio, temporary and accidental in/dwelling of the Word of God in the man in a manner the Spirit dwelt in prophets. The effect of the in/dwelling of the Word in a man born of Mary (or the dwelling itself) can be taken as prophetal inspiration, animation, consisting of mere external strengthening of the man and empowering him for his and Divine activity, nevertheless, man’s vital and, and as it seems operative, principle is his soul. Based on De Trinitate, Divine Sonship or filiation and “deification” of man born of Mary, according to Photinus, seems to be due to the fact that the non-subsisting Word of God – a part of God’s powers – dwells in him, inspiring or animating him by strengthening him and empowering him for divine activity. According to Hilary, Photinus denies pre-existence of the Word, that is, the Son, Christ so he cannot even be the co-Creator of the world. He becomes existent, that is, subsistent only through the Incarnation and birth of Mary. For Hilary, Photinus’ adoptionist position is clear: the man is assumed into the Son and into the God. According to Hilary, in Photinus’ doctrine there is no place for the real Incarnation of the true Son of God. Hilary’s interpretation of Photinus’ understanding of Jesus Christ, the Son, is that he is not the Word made flesh, nor he is one and the same both God and Man. For Hilary Jesus Christ or Son of God as Photinus understands him is just someone like a prophet (a man) inspired, that is empowered by a Word of God dwelling in him – by a part of God’s powers – for divine activity; ultimately, Hilary reduces him to a mere man, to a creature. The fourth part points out that opinions expressed in the scholarship – based exclusively on the Book Ten of Hilary’s De Trinitate – according to which Photinus, motivated by soteriology, insisted on the wholeness of Jesus’ humanity that is on the fact that Jesus Christ had a human soul, should be taken relatively. To conclude, on the basis of Book Ten of De Trinitate Photinus insisted on the wholeness of the humanity of Jesus Christ, that is, on his possessing of the human soul, just to the extent which he held that he was a mere man (in whom the non-subsistent Word of God dwelt as a Spirit in prophets).