Uh oh. This is one of those topics that gets heated around here.
CKC is either Continental Kennel Club or Canadian Kennel Club (usually the first).
There is always a chance that a dog registered with any kennel club is not purebred but that increases with Continental. There are a few good CKC breeders but it isn't a good registry in general. I would most likely never buy a Continental registered dog (and never to breed or show).
Canadian Kennel Club (which doesn't apply to you) is fine also.