Paul and Barnabas go to Lystra in Acts 14, and God performs a miracle:
Now at Lystra there was a man sitting who could not use his feet. He was crippled from birth and had never walked. He listened to Paul speaking. And Paul, looking intently at him and seeing that he had faith to be made well, said in a loud voice, “Stand upright on your feet.” And he sprang up and began walking. Acts 14:8-10
The crowds literally go wild:
And when the crowds saw what Paul had done, they lifted up their voices, saying in Lycaonian, “The gods have come down to us in the likeness of men!” Act 14:11
Shortly after, the people of Lystra respond differently:
But Jews came from Antioch and Iconium, and having persuaded the crowds, they stoned Paul and dragged him out of the city, supposing that he was dead. Act 14:19
I believe God has the sovereign authority and ability to do whatever he wants with his creation, including miracles of physical healing in our present day. I don’t believe they are the norm, however.
And the above example from Acts 14 demonstrates that the presence of miracles is not enough to change hearts. Those people in Lystra observed the healing, proclaimed Paul a god, and then tried to kill him!
So, I pray, earnestly, for my wife that God would keep her cancer at bay. But neither she nor I need a miracle to believe that God is who he says he is. God did that work when he gave us faith.