Martin Luther, preaching on Christmas Day on Luke 2:15-20 (the part of the Christmas story where the shepherds go to Bethlehem and find Jesus), said:
The first and principal fruit of the power of the Word is faith. For had not these shepherds believed the angel they would never have gone to Bethlehem, they would moreover never have done one of the things related of them in this Gospel.
One, however, might say; Yes, I would also gladly believe if an angel thus from heaven were to preach to me. This is very foreign to the subject. Whoever does not receive the Word for its own sake, will never receive it for the sake of the preacher, even if all the angels preached it to him. And he who receives it because of the preacher does not believe in the Word, neither in God through the Word, but he believes the preacher and in the preacher. Hence the faith of such persons does not last long. But whoever believes the Word, does not care who the person is that speaks the Word, and neither will he honor the Word for the sake of the person; but on the contrary, he honors the person because of the Word, and always subordinates the person to the Word. And if the preacher perishes, or even falls from his faith and preaches differently, he will forsake the person of the preacher rather than the Word of God. He abides by what he has heard, although the person of the preacher may be what he will, and come and go as he may.
The tru
e difference between godly faith and human faith consists also in this, that human faith cleaves to the person of the preacher, believes, trusts and honors the Word for the sake of him who spake it. But godly faith, on the other hand, cleaves to the Word, which is God himself; he believes, trusts and honors the Word, not because of him who preaches it; but because he feels it so surely the truth that no one can ever turn him again from it, even if the same preacher were to try to do it.
Luther, Martin Sermons of Martin Luther, volume 1 (Baker Book House) 162-163