Let’s give it another try.  John accepts the categorical imperative.  John chooses to lie.  Because he accepts the categorical imperative, by lying John is logically implying that it is possible for everyone to lie all the time.  What happens if everyone lies all the time?  Suppose (for the purposes of the thought experiment, hypothetically) that you live in a world where everyone lies all the time.  Now John tells you his lie.  Do you believe him?