Why does God let men preach?
This is the best and most useful exercise in humility, when (God) accustoms us to obey his Word, even though it be preached through men like us and sometimes even by those of lower worth than we. If he spoke from heaven, it would not be surprising if his sacred oracles were to be reverently received without delay by the ears and minds of all. For who would not dread the presence of His power? Who would not be stricken down at the sight of such great majesty? Who would not be confounded at such boundless splendor? But when a puny man risen from the dust speaks in God’s name, at this point we best evidence our piety and obedience toward God if we show ourselves teachable toward his minister, although he excels us in nothing.
–John Calvin (Institutes 4.3.1)