Auguste Kerckhoffs is probably as well-known as any self-respecting cryptographer would care to be. In 1883 he published La Cryptographie Militaire, in which he explored the current state-of-the-art in military cryptography, as well as a plea for significant improvements in French practice.
The work included many pieces of practical advice and rules of thumb, including six principles of practical cipher design. Several of these design considerations are no longer relevant thanks to modern technology, however his second axiom, now known as Kerckhoff's principle, remains indispensable:
"The system must not require secrecy and can be stolen by the enemy without causing trouble"
There are two things worth considering about this statement.
First, it extends well beyond ciphers and codes, and should be kept top-of-mind in the design of organizational structure, product features, business processes, as well as personal relationships with family, friends, and strangers.
Second, it's a subtle directive. One worth thinking about with care. Kerckhoffs wasn't suggesting absolute secrecy or transparency. What the principle demands is that a system should remain secure even when the enemy has a copy.
Your opponents already know the details of your system. And they always will. Start with that assumption.