Class,
We don't even have to worry about John wanting to tell the truth in the future. The fact that you won't believe him when he tells his first lie is enough (and I would hope that all of us would not believe John, if everyone lies all the time). We've reached the contradiction. Think about it. What is the point of telling a lie?
Can you see the contradiction? If not, just focus on the question I just asked, What is the point of telling a lie?