John 14:5 - Albert Barnes' Notes on the Bible

We know not whither thou goest - Though Jesus had so often told them of his approaching death and resurrection, yet it seems they did not understand him, nor did they fully comprehend him until after his resurrection. See Luke 24:21. They entertained the common notions of a temporal kingdom; they supposed still that he was to be an earthly prince and leader, and they did not comprehend the reason why he should die. Thomas confessed his ignorance, and the Saviour again patiently explained his meaning. All this shows the difficulty of believing when the mind is full of prejudice and of contrary opinions. If Thomas had laid aside his previous opinions - had he been willing to receive the truth as Jesus plainly spoke it, there would have been no difficulty. Faith would have been an easy and natural exercise of the mind. And so with the sinner. If he were willing to receive the plain and unequivocal doctrines of the Bible, there would be no difficulty; but his mind is full of opposite opinions and plans, occupied with errors and vanities, and these are the reasons, and the only reasons, why he is not a Christian. Yet who would say that, after the plain instructions of Jesus, Thomas might not have understood him? And who will dare to say that any sinner may not lay aside his prejudices and improper views, and receive the plain and simple teaching of the Bible?