In his “Summary of Doctrine Concerning the Ministry of the Word and the Sacraments,” Calvin articulates the idea of union and communion with Christ through the means of grace.
The end of the whole Gospel ministry is that God … communicate Christ to us who are disunited by sin and hence ruined, that we may from him enjoy eternal life; that in a word all heavenly treasures be so applied to us that they be no less ours than Christ’s himself.
We believe this communication to be mystical, and incomprehensible to human reason, and Spiritual, since it is effected by the Holy Spirit [by whom] he joins us to Christ our Head, not in an imaginary way, but most powerfully and truly, so that we become flesh of his flesh and bone of his bone, and from his vivifying flesh he transfuses eternal life into us.
To effect this union, the Holy Spirit uses a double instrument, the preaching of the Word and the administration of the sacraments.
When we say that the Holy Spirit uses an external minister as instrument, we mean this: both in the preaching of the Word and in the use of the sacraments, there are two ministers, who have distinct offices. The external minister administers the vocal word, and the sacred signs which are external, earthly and fallible. But the internal minister, who is the Holy Spirit, freely works internally, while by his secret virtue he effects in the hearts of whomsoever he will their union with Christ through one faith. This union is a thing internal, heavenly and indestructible.
In the preaching of the Word, the external minister holds forth the vocal word, and it is received by the ears. The internal minister, the Holy Spirit, truly communicates the thing proclaimed through the Word, that is Christ…. so that it is not necessary that Christ or for that matter his Word be received through the organs of the body, but the Holy Spirit effects this union by his secret virtue, by creating faith in us, by which he makes us living members of Christ, true God and true man.
 Jean Calvin, Theological Treatises, ed. J.K.S. Reid (Louisville, KY: Westminster John Knox, 2006), 170–77.