John Duncan was a minister in the Free Church of Scotland and a missionary to the Jews in Hungary. John Piper quotes John Duncan here:
"As long as I am told that I must come to God, and that I can come, I am left to suppose that some good thing, or some power of good remains in me, and I arrogate to myself that which belongs to Jehovah. The creature is exalted, and God is robbed of His glory.
If, on the other hand, I am told that I cannot come to God, but not also that I must come, I am left to rest contented at a distance from God, I am not responsible for my rebellion, and God Jehovah is not my God.
But if we preach that sinners can't come, and yet must come, then is the honour of God vindicated, and the sinner is shut up. Man must be so shut up that he must come to Christ, and yet know that he cannot. He must come to Christ, or he will look to another, when there is no other to whom he may come; he cannot come, or he will look to himself.
This is the gospel vice, to shut up men to the faith. Some grasp at one limb of the vice and some at the other, leaving the sinner open - but when a man is shut up that he must and cannot, he is shut up to the faith - shut up to the faith, and then would he be shut up in the faith. God is declared to be Jehovah, and the sinner is made willing to be saved by Him, in His own way, as sovereign in His grace."