Saint Cyril of Alexandrias formula “union by hypostasis” is the basis of the dogmatic teaching of the Ecumenical Councils and of all the normative Eastern Fathers. Through it, Saint Cyril defended the teaching about the unity of the person of Christ against Nestorius, but also against the Arians and the Apollinarians. Saint Cyrils starting point is the scriptural passage from John 1, 14: “The Word became flesh”. Saint Cyril first sees the hypostasis of the Word becoming man and assuming a nature that has no subsistence of its own. At the same time, Saint Cyril emphasizes the fact that human nature is not absorbed in divinity, but remains whole after union with the Word, because this union involves neither mixture nor confusion. The separate natures have merged into one true unity, but the One is of both, Christ and the Son. But this unity is real, being given in the person of Christ by the hypostasis of the Word Who, through the hypostatic union with human nature, is One and the Same. Saint Cyril speaks of the humanity of the Word which became the Words own through the Hypostatic Union. The True Union or the Hypostatic Union corresponds to the reality of the body made proper by the Word. The fact that the Word makes its own humanity through this union, shows the unity of the person of Christ, even if two natures are admitted after the union. The threads correspond to their own works and wills.