Do people with celiac disease always have the symptoms? can you have the disease and not show any symptoms?

It varies: Symptoms can vary from mild GI discomfort to severe symptoms and an itchy rash, not everyone manifests the disease the same way.

Varies a lot: True CD will eventually show some symptoms if the person is not gluten free.These can vary from mild cramping & diarrhea to chronic abdominal pain, diarrhea & catastrophic weight loss. Some may inadvertently be living gluten free based on prior food reactions, but the difficulty in avoiding gluten leads to symptoms & diagnosis.I probably had it a decade before I figured it out.