Albert Barnes on Mark 9:23

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. 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 favour of him that believes; and if thou canst believe, this will be done. God will do nothing in our favour 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.