Does having more faith improve cancer outcomes?

There is no good evidence for it.

Research on religion, spirituality and cancer measures how people feel and function: quality of life, mood, distress. It does not measure whether a tumor responds or how long someone lives. The National Cancer Institute's review adds a caution. The reported links between religious involvement and health have been criticized as weak and inconsistent.

One finding is worth knowing if someone has told you to pray harder or attend more often. In a national survey of 361 pairs of survivors and caregivers, a sense of peace was strongly linked to mental health. The faith score, meaning religiousness, was not linked to physical or mental wellbeing at all. In a study of 418 breast cancer patients, a greater sense of meaning and peace went with less depression over 12 months. Higher religiousness predicted more depression, not less.

So the part that seems to help is a sense of meaning and peace. That is not something a person can produce on demand by trying harder.

Catholic teaching says something similar in its own terms. The Catechism states plainly that "even the most intense prayers do not always obtain the healing of all illnesses."

If this is being said to you over and over, a hospital chaplain can help. They can often reach a family member better than you can.