Quote from Martin Luther agreeing that one must be sent from God, as the prophets were, in order to be believed:

"Now, Christ interprets his own words. He says that he is the door to the sheep, but all the others who came before him, that is, those who were not sent by God as the prophets were, but came of themselves, uncommissioned, are thieves and murderers; they steal his honor from God and strangle human souls by their false doctrines."

Sermon for Pentecost Tuesday; John 10:1-11
A Sermon by Martin Luther; taken from his Church Postil, 1523. THE MARKS OF FALSE PREACHERS.