The Holy Spirit as the “Vicar” of Christ

In Matthew 16:18, Jesus promised he would build his church. How can this be, if he rose and ascended before its formal birth at Pentecost in the Acts of the Apostles (ch.2)? The book itself helps us here as the “acts” of the Apostles were carried out through the power of the Holy Spirit. The book fulfills Christ’s promise in Acts 1:8 that the disciples would be witnesses everywhere geographically and to everyone ethnically once the Holy Spirit had come upon them. As the book unfolds, so the Church takes shape through the Spirit-empowered preaching of the gospel. Still, how does Jesus build his church if he is absent bodily? Simply, through the Holy Spirit.
In this way, we identify the resurrected and ascended Christ with the Holy Spirit through their joint activity in the progress of the church not in the sense that they were fused as one person in the loss of personal distinction. We see such identity in 1 Corinthians 15:45 where Paul calls Christ the last Adam as a “quickening” or life-giving “Spirit.” In 2 Corinthians 3:17 Paul also declares, “the Lord is that Spirit” who removes the veil of blindness to the Old Testament. Other verses such as Romans 8:9, Philippians 1:9, and 1 Peter 1:1 utilize the phrase the “Spirit of Christ”, which was used interchangeably with the Holy Spirit.
This identity between Christ and the Spirit existed in such 17th century Puritans as John Owen. In Pneumatologia: Or, Discourse Concerning the Holy Spirit (1674; Works, vol. 3), he states that the sending the Holy Spirit was absolutely essential to the work to which Jesus called the Apostles as his “witnesses” in Acts 1:8. “Here lay the foundation of the church,” claims Owen. On the Spirit’s “presence and assistance alone depended the whole success of their ministry in the world.” By the promise of the Holy Spirit, Christ “founded the church, and by it he built it up.” Very simply, without the Spirit to build the church, it would come to an end: “no dispensation of the Spirit, no church.”
Owen also notes that is “the Holy Spirit who supplies the bodily absence of Christ, and by him doth he accomplish all his promises to the church.” The Spirit is the “vicar of Christ” (Vicarium Christi), the subsitiute who “represents his person, and dischargeth his promised work.” No doubt, Owen had in mind the pompous claims of popes [e.g. Innocent III  (1198-1216) and Boniface VIII (1294-1303)] for the title the “Vicar of Christ.” Such a role belongs to the Spirit alone. He attests: 
In and by [the Holy Spirit] [Christ] is present with his disciples in their ministry and their assemblies . . . The Lord Jesus hath told us that his presence with us by his Spirit is better and more expedient for us than the continuance of his bodily presence. . . As [the Holy Spirit] represents the person and supplies the room and place of Jesus Christ, so he worketh and effecteth whatever the Lord Christ hath taken upon himself to work and effect towards his disciples.
Parallel to Owen, Sinclair Ferguson (The Holy Spirit) more recently notes, “Christ on his ascension came into such complete possession of the Spirit who had sustained him throughout his ministry that economically the resurrected Christ and the Spirit are one to us.” The risen Christ is a life-giving Spirit, “the source of our resurrection existence” and, according to  2 Cor 3:17–18, “the Spirit has been ‘imprinted’ with the character of Christ.” This does not nescessitate an “ontological fusion” obliterating “the distinction in Spirit” but “a complete intimacy of relationship between Jesus and the Spirit.” Similarly, Richard Gaffin identifies the “unity of Christ and the Spirit in saving activity” for a “functional, dynamic identity” of “essence and power” (Resurrection and Redemption).
What a glorious thought! Jesus Christ right now through the Spirit is building his Church to its completion. The very gates of Hades, the realm of the dead, cannot hold back the progress of this work (Matt. 16:18). Jesus guaranteed the victory, this trampling of the gates of death, through his own death and resurrection (Matt. 16:21).  Death as the wages of sin could not hold him, because he never sinned. The risen ascended Savior could send the promised Spirit to build the church, through whom he would not leave his disciples as comfortless orphans. He promised, “I will come to you,” and he did through the “Spirit of truth” (John 14:17-18).  The work of the church did not depend on the disciples then and it does not depend on us now. What a relief!  What a caution (Acts 2:47; 1 Cor. 4:7).