See how different Christ is from his successors, although they all would wish to be his vicars. A man is a vicar only when his superior is absent. If the pope rules, while Christ is absent and does not dwell in his heart, what else is he but a vicar of Christ? What is the church under such a vicar but a mass of people without Christ? Indeed, what is such a vicar but an antichrist and an idol? How much more properly did the apostles call themselves servants of the present Christ and not vicars of an absent Christ?from “An Open Letter to Pope Leo X” (1520) in Martin Luther’s Basic Theological Writings. Ed. Tim Lull (1989), 594.
[Polemically stated, to be sure, but worth bearing in mind the distinction between serving a present Christ and standing in for an absent Christ.]