Who then is John Piper?: "Successful" preaching as the work of the Holy Spirit, rather than of man

When Your Preacher Is Not John Piper – The Gospel Coalition Blog

Many who have had the privilege of hearing John Piper preach in person...  might leave wondering if they were just in the presence of a figure who will be talked about in future centuries.
Then they go back to their home church, where several things are different, including the preaching. Thankfully, the gospel is still proclaimed. In fact, the sermons are thoroughly biblical, but the ability of their regular preacher simply does not measure up to the phenomenal preaching they recently heard.

Ultimately, it is the messenger's job to deliver the message of the One who sent him or her.  Salvation isn't dependent on the oratory ability of the messenger, but on the power of the Holy Spirit to convict sinners of the need for repentance.  This isn't meant to disparage John Piper or any other "celebrity preacher", but rather focus the glory on the One to who it is due.

This reminds me of a passage in The Cross and the Switchblade where Stagecoach, the leader of the Chaplains gang, tells David Wilkerson that his message was "coming through" to which Wilkerson replies that it wasn't he "who was coming through but the Holy Spirit" (p. 70). And in the end, that's who should be "coming through" in any sermon, regardless of the popularity, charisma, speaking ability, or whatever (or lack thereof) of the person preaching it.