In E.M. Bounds, The Essentials of Prayer, while discussing the need for man to give his whole self to God in prayer Bounds states this,
Many a praying preacher has been greatly helped just at this point. The unction of the Holy One which comes upon the preacher invigorates the mind, loosens up thought and gives utterance. This is the explanation of former days when men of very limited education had such wonderful liberty of the Spirit in praying and in preaching. Their thoughts flowed as a stream of water. Their entire intellectual machinery felt the impulse of the divine Spirit's gracious influences.
Imagine the uneducated leaning on the Lord in fully-surrendered prayer and finding in Him alone the strength and wisdom to preach powerfully.