"If they were religious enough their cancer would be cured"

That makes God look disgusting.

Well, it doesn't make God look like anything he isn't. It makes the speaker look bad, though; he appears to think that he has God on puppet strings whereby he is bound by a promise he never made, to give us everything we request.

God is not our cosmic puppet. He doesn't pop out of a tardis (ex machina) to make our lives good in whatever way we have specified. Snap my fingers, you do my bidding, O God, because you promised.

You're right, God often answers, "No". He told Paul no when the latter had appealed to him to cure his pain. After three such requests, God said "no" and Paul submitted to his will. Unlike the name-it-and-claim-it boobies who think God owes them a cure.