[H]e [the preacher] should not speak uncertainly as if he were giving out comments of his own, but he should be able to speak out confidently without hesitation in the name of God; just as Jeremiah in this passage [Jer 1] demands to be heard because, as he declares, God has put his words in his mouth.
We can be sure that whatever comes from man’s own cleverness may be ignored. God demands for himself alone the honor of being heard in his church (as I said yesterday). Hence it follows that none should be recognized as servants of God, none should be counted just and faithful prophets or teachers, unless God is speaking through them, unless they invent nothing by themselves and teach nothing by their own will, but preach only what God commands. — John Calvin