The spec currently says:

When building the Configuration object, if the ValidationProviderResolver either fails or if the expected provider is not found, a ValidationException is raised.

However, not in all situations an exception can be raised at the point when the Configuration is built. For example if the provider is specified in validation.xml and the default bootstrapping is used the exception cannot be raised until buildValidatorFactory is called. For this reason the spec should say:

When bootstrapping a Bean Validation ValidatorFactory, if the
ValidationProviderResolver either fails or if the expected provider is
not found, a ValidationException is raised.