Two views of why Jesus describes the Canaanite woman’s faith as great:
Though never before having told someone that their faith is great, Jesus does say this to this woman in Matthew 15:28: not for persisting, but rather for trusting in the rejected Messiah, treated by Israel as a crumb. What the unbelieving children refuse, she takes as her riches, glorying in the cross of Christ.
The woman’s faith is shown in her confessing the truth of Jesus’ claims as a basis for healing. He describes a faith that is shown in a miracle: a miracle faith. Whether it was a saving faith is not known, but like many of Jesus’ miracles, this shows the path for saving faith and the hope to which it may look forward.
Interpretation 1:
Despite never before directly telling someone that their faith is great, Jesus does directly say this to the Canaanite woman in Matthew 15:28. Why? Not because she persisted but rather because she believed in the suffering Messiah whom Israel treats as a crumb. That which the unbelieving children disdain, she embraces as her riches. She glories in the cross of Christ.1
Interpretation 2:
The woman’s faith is evident from her words, as she acknowledges the truth of Jesus’ claims and holds him to it as the very means to acquiring healing. What Jesus therefore describes here is a faith that is now revealed through a miracle—a miracle faith. Whether this was also a saving faith on her part remains an open question. In any case, this event, like so many of Jesus’ miracles, is meant to show the path that saving faith must take, thereby indicating the hope to which it may look forward.2
28 Alors Jésus lui dit: Femme, ta foi est grande; qu'il te soit fait comme tu veux. Et, à l'heure même, sa fille fut guérie.