There is a fine line between having faith in a genuine promise of God and having faith in something that he has not promised to do, as illustrated in the recent tragic case of Madeleine Newmann, whose mother Leilani has been convicted of reckless homicide for trusting in faith healing instead of seeking medical assistance for Madeleine’s treatable diabetic condition. Leilani’s prayers were unsuccessful, and Madeleine died.
Medicine is still in many ways an imprecise science. There are many diseases and conditions which doctors simply do not know how to treat, and there are many occasions on which medical diagnoses turn out to be wrong, or changes happen which doctors are unable to explain: hence there are numerous cases of “miracle cures”. In the New Testament, Jesus cures a number of people who are suffering illnesses but does not cure others, so it is reasonable for people of faith to believe that God may bring about a cure.
Nevertheless, most people with a Christian worldview would agree that whilst God can provide miracles, he has also provided doctors. In the same way that God normally provides food via the local supermarket rather than by making it miraculously appear in the kitchen cupboards, God normally provides medical care by means of the local doctor, rather than by some miraculous healing.