“Rejoice! And I say it again, rejoice!” The notion of joy is altogether fundamental to Christianity, which by its very nature is and claims to be evangelium, gospel, joyful news. Yet it is true: the world mistrusts the gospel, mistrusts Christs on this very point, and it abandons the Church because Christianity would deprive people of joy by imposing its endless demands and injunctions.
Granted, this much is correct: the joy of Christ cannot be recognized as easily as the superficial pleasure that may result from some entertainment. Still, it would be wrong to interpret the exhortation, “Rejoice in the Lord”, as meaning, “Rejoice, but in the Lord”, as if the added clause would take back what was said in the first place. No, we simply read, “Rejoice in the Lord”, because the Apostle obviously believes that all true joy is joy in the Lord and that there can be no true joy at all without the Lord.
And it remains also true that every “joy” that occurs in disregard of the Lord or in opposition to him never satisfies but rather drives a person ever deeper into a vortex, which in the end he himself would indeed no longer find enjoyable at all. Thus we are told that only through Christ did real joy appear and that in our life, in the last analysis, nothing matters more than coming to recognize and to understand Christ, the God of grace, the light and the joy of the world. Only then will our joy be true, when it no longer relies on things that can be snatched away from us and can perish, but when it is rooted in the innermost core of our existence, which no power in all the world is able to take away from us. And every outward loss ought to become for us a pathway into these innermost realms and to prepare us ever more for our true life.
From: Dogma und Verkundigung, pp. 376f.