The greatest Puritan theologian and contemporary of Bunyan,
John Owen, when asked by King Charles why he, a great scholar, went to hear an uneducated tinker preach said, "I would willingly exchange my learning for the tinker's power of touching men's hearts."
From:
To Live Upon God that Is Invisible :: Desiring God Christian Resource Library
Also from the same link are quotes besides J. Owen that are worth mentioning:
This is extraordinary for a man with no formal education. He knew neither Greek nor Hebrew and had no theological degrees. This was such and offense even in his own day that his pastor,
John Burton, came to his defense, writing a foreword for his first book in 1656 (when he was 28): "This man is not chosen out of an earthly but out of the heavenly university, the Church of Christ . . . . He hath through grace taken these three heavenly degrees, to wit, union with Christ, the anointing of the Spirit, and experiences of the temptations of Satan, which do more fit a man for that mighty work of preaching the Gospel than all university learning and degrees that can be had."
George Whitefield said of The Pilgrim's Progress, "It smells of the prison. It was written when the author was confined in Bedford jail. And ministers never write or preach so well as when under the cross: the Spirit of Christ and of Glory then rests upon them."
And
He poured over his English Bible, which he had most of the time. This is why he can say of his writings, "I have not for these things fished in other men's waters; my Bible and Concordance are my only library in my writings."
Charles Spurgeon put it like this: "He had studied our Authorized Version . . . till his whole being was saturated with Scripture; and though his writings . . . continually make us feel and say, 'Why, this man is a living Bible!' Prick him anywhere; and you will find that his blood is Bibline, the very essence of the Bible flows from him. He cannot speak with out quoting a text, for his soul is full of the Word of God."