John Gill Commentary Matthew 15:28

"Then Jesus answered and said unto her, O woman, great is thy faith: be it done unto thee even as thou wilt. And her daughter was healed from that hour." — Matthew 15:28 (ASV)

Then Jesus answered, and said to her
As one surprised at the strength of her faith, and the clearness and justness of her pious reasoning; and not concealing himself, and the designs of grace, any longer from her, breaks out in great admiration of her, saying,

O woman, great is your faith!
He seems surprised, that she, a woman, and a poor Gentile, should express such strong faith in him; calling him Lord, owning him to be the Messiah, worshipping him as God, believing him able to do what could not be done by human art; and though she met with such repulses, and even called a dog, yet still continued importunate with him, believing she should succeed:

be it to you even as you wish ;
let your daughter be healed, as you desire, and in the way, and at the very time you would have it:

and her daughter was made whole from that very hour :
power went forth from Christ, and dispossessed the devil; so that when she came home, as Mark observes, she found her daughter lying on the bed, quiet, and easy, and perfectly well. The conduct of our Lord towards this woman, and her behaviour under it, do, in a very lively manner, represent the methods which God sometimes takes with his people, when they apply to him in their distress; and the nature and actings of their faith upon him: as she, when she first applied to Christ for mercy and help, had not a word of answer given her; so sometimes they cry, and the Lord turns a deaf ear, or seems not to hear, and, in their apprehension of things, has covered himself with a cloud, that their prayer should not pass through; however, an immediate answer is not returned; yea, when others interpose on their behalf, and entreat for them, yet no favourable answer is returned, as was not by Christ to his disciples, when they implored him on this woman's account: and yet, notwithstanding all this, like her, they are not discouraged, but persistently appeal to the throne of grace with renewed requests, acknowledge that the worst of names and characters belong to them: that they are unworthy of the least of mercies, and should be content with the crumbs of divine favour, but cannot go away without a blessing; they lay hold on every word of God, and hastily catch at it, and improve everything in their own favour, that faith can come at, and so, in the issue, succeed in their requests.

Effectual, fervent, and importunate prayer, the prayer of faith avails much with God.