To justify the Reformation against Catholicism, Calvin as a reformer of the second generation, had to establish two basic principles: the "certainty of salvation" and "morality of salvation," in other words, justification" and sanctification". To do this, Calvin took the notion of union or of communion with Christ as the central idea. Conceming the subject of union with Christ, my thesis aims to solve two problems: First, what is the nature of union with Christ in Calvin's theology ? Second, Christ is in heaven and is at the right hand of God. There is a distance between Christ and believer. If so, how can they unite together ? What is the meaning of the union in this distance ? The nature of this union is Christological. So we can find an analogy between the "union of two natures in Christ" and the "union of Christ and a believer". We applied the two concepts, "communicatio idiomatum" and "extra calvinisticum" to the notion of the union between Christ and a believer. The "communicatio idiomatum" makes it possible to unite Christ with the believer and opens the possibility of exchange of the person between them, and ensures the certainty of salvation. The "extra calvinisticum" claims that there is still the divine transcendence in the union between Christ and a believer. By the Holy Spirit, they are united, but this union is linked to the divine transcendence. The presence of the Christ through the Holy Spirit can be called "the absent presence", which helps us to stay in the eschatological hope.The perfect union is eschatological.
