Behind glass, above reach, out of direct sunlight, and with each piece properly retained so it cannot fall. A closed cabinet protects the knives from dust, humidity and fingerprints, and protects everyone else from an open display of sharp objects at chest height.
A glass-fronted cabinet solves several problems at once: dust, humidity swings, casual handling and safety. Open wall displays look excellent and expose blades to all four.
If the cabinet locks, better still. That covers children and visitors without you having to think about it.
Keep it out of direct sunlight, which fades handle materials and degrades leather. LED lighting inside a cabinet is fine because it produces very little heat, unlike halogen.
Mount each piece so it cannot slide or fall. A knife dropping inside a cabinet can damage itself and everything below it. Avoid unknown foams, some of which off-gas and mark steel over time.
Height and a locked door, together. A display is exactly the kind of thing a curious child works out how to reach.