This essay examines the concept of union with Christ in Calvin’s theology. I focus on Calvin’s commentary on the Gospel of John 13-17 in which Calvin employs the organic aspects of engrafting in order to develop the doctrine of union with Christ. Although Calvin fully acknowledges the mystic nature of this union, he carefully prevents his doctrine from falling into that of mysticism. I explicate the different meanings with which Calvin uses the image of engrafting and the firm limits he places around it. The result shows how Calvin’s exegesis feeds into the development of a doctrine.