If you associate with the Christian, you can sense the difference. The difference is real, it is genuine, because the Christian has a purpose that is bigger than life, and is even bigger than death. That purpose affects the way that the Christian does things. It affects the way that the Christians treats people. It affects every aspect of the Christian's life.
People see the purpose in the joy.
And it is in that purpose that the Christian finds joy that cannot be destroyed. Having purpose in Christ produces a joy that cannot be destroyed. It can only be found in Christ. I don't desire for people to claim to be Christian if they don't repent. It is only with faith, conversion, and commitment that we find the joy that comes through serving Christ's purposes.