Why is the Gospel Powerful?

Quoting Klemet Preus from his excellent essay "THE THEOLOGY OF THE CHURCH GROWTH MOVEMENT: An Evaluation of Kent Hunter's Confessions":

One of Luther's most significant contributions to theology, built upon his doctrine of justification, is his understanding of the inherent power of the gospel. The gospel does not become powerful when and if something is added. It is powerful always because Jesus is both its content and its administrator. Every false teaching can be evaluated and described in terms of what that false teaching tries to add to the gospel to make it work. The word becomes powerful when it is preached by a spirit-filled preacher or when the message is "anointed" by the spirit (Wesleyanism, Holiness Movements, Pentecostalism). The word becomes powerful when the sovereign God wills it or when preached to the elect (Calvinism). The word becomes powerful when placed into the teaching office (Romanism.) The word becomes powerful when combined with the willing heart (Arminianism). The word becomes powerful when the "meaning of the words," combines with the "power with which these words are spoken," and the "existential reception of the content" and the "correlation of these" into a "constellation in which the words become the Word" (Paul Tillich). The word becomes powerful in an "I/thou encounter" (Barth). The word becomes powerful "when we get out of God's way," or when placed into the hands of a church that has accepted the "mission paradigm" or "thinks like a missionary," or that has become a "great commission church" (Hunter and the Church Growth Movement). To Luther, and we might add, to the Holy Spirit, the word is powerful because in it Jesus speaks and forgives. "At whatever hour, then, God's word is taught, preached, heard, read or meditated upon, there the person, day and work are sanctified thereby, . . . because of the Word which makes saints of us all" (LC I, 91).