The Key is emptiness of self. The emptier, the more Christ is present, and speaks with power.
“This is the function in persona Christi of the priest: to render present, in the confusion and disorientation of our times, the light of the Word of God, the light that is Christ himself in this our world. Hence the priest does not teach his own ideas, a philosophy that he himself has invented, has found and that pleases him; the priest does not speak of himself, does not speak by himself, to create perhaps admirers or his own party; he does not say his own things, his own inventions, but, in the confusion of all the philosophies, the priest teaches in the name of Christ present, he proposes the truth that is Christ himself, his word, his way of living and of going forward. True for the priest is what Christ said of himself: "My teaching is not mine" (John 7:16); that is, Christ does not propose himself, but, as Son, is the voice, the word of the Father. The priest must also speak and act like this: "My doctrine is not mine, I do not propagate my ideas or what pleases me, but I am the mouth and heart of Christ and make present this unique and common doctrine, which the universal Church has created and which creates eternal life."