See this passage explained in the notes at Matthew 17:14-21.
Questioning with them - Debating with the disciples, and attempting to confound them. This he saw as he came down from the mount. In his absence they had taken occasion to attempt to perplex and confound his followers.
Were greatly amazed - Were astonished and surprised at his sudden appearance among them.
Saluted him - Received him with the customary marks of affection and respect. It is probable that this was not by any “formal” manner of salutation, but by the “rush” of the multitude, and by hailing him as the Messiah.
What question ye? - What is the subject of your inquiry or debate with the disciples?
A dumb spirit - A spirit which deprived his son of the power of speaking.
And wheresoever - In whatever place - at home or abroad, alone or in public.
He teareth him - He rends, distracts, or throws him into convulsions.
He foameth - At the mouth, like a mad animal. Among us these would all be considered as marks of violent derangement or madness.
And pineth away - Becomes thin, haggard, and emaciated. This was the effect of the violence of his struggles, and perhaps of the want of food.
If thou canst do any thing - I have brought him to the disciples, and they could not help him. If thou canst do anything, have compassion.
If thou canst believe - This was an answer to the request, and there was a reference in the answer to the “doubt” in the man’s mind about the power of Jesus. “I” can help him. If thou” canst believe,” it shall be done. Jesus here demanded “faith” or confidence in his power of healing. His design here is to show the man that the difficulty in the case was not in the want of “power” on his part, but in the want of “faith” in the man; in other words, to rebuke him for having “doubted” at all whether he “could” heal him. So he demands faith of every sinner that comes to him, and none that come without “confidence” in him can obtain the blessing.
All things are possible to him that believeth - All things can be effected or accomplished - to wit, by God - in favor of him that believes, and if thou canst believe, this will be done. God will do nothing in our favor without faith. It is right that we should have confidence in him; and if we “have” confidence, it is easy for him to help us, and he willingly does it. In our weakness, then, we should go to God our Saviour; and though we have no strength, yet “he” can aid us, and he will make all things easy for us.
Said with tears - The man felt the implied rebuke in the Saviour’s language; and feeling grieved that he should be thought to be destitute of faith, and feeling deeply for the welfare of his afflicted son, he wept. Nothing can be more touching or natural than this. An anxious father, distressed at the condition of his son, having applied to the disciples in vain, now coming to the Saviour; and not having full confidence that he had the proper qualification to be aided, he wept. Any man would have wept in his condition, nor would the Saviour turn the weeping suppliant away.
I believe - I have faith. I do put confidence in thee, though I know that my faith is not as strong as it should be.
Lord - This word here signifies merely “master,” or “sir,” as it does often in the New Testament. We have no evidence that he had any knowledge of the divine nature of the Saviour, and he applied the word, probably, as he would have done to any other teacher or worker of miracles.
Help thou mine unbelief - Supply thou the defects of my faith. Give me strength and grace to put “entire” confidence in thee. Everyone who comes to the Saviour for help has need of offering this prayer. In our unbelief and our doubts we need his aid, nor shall we ever put sufficient reliance on him without his gracious help.