Adam Clarke on Luke 6:39

Can the blind lead the blind? - This appears to have been a general proverb, and to signify that a man cannot teach what he does not understand. This is strictly true in spiritual matters. A man who is not illuminated from above is utterly incapable of judging concerning spiritual things, and wholly unfit to be a guide to others. Is it possible that a person who is enveloped with the thickest darkness should dare either to judge of the state of others, or attempt to lead them in that path of which he is totally ignorant! If he do, must not his judgment be rashness, and his teaching folly? - and does he not endanger his own soul, and run the risk of falling into the ditch of perdition himself, together with the unhappy objects of his religious instruction?