An open declaration of an opinion or belief is a profession. If you announce that you believe the earth is flat, your profession of this opinion might cause some giggling among your well-educated friends.
Careers that require specialized training or advanced degrees are considered professions. Your parents may want you to go into a profession like the practice of law, but you dream of being a rodeo clown. The collective group of people practicing a profession is also called profession. The medical profession doesn't know what to make of that thing growing out of your foot. You are a medical mystery!