What is the real meaning of the church for Christian life? If we confess Christian faith, the church is one of the twelve articles, which implies that the church is rather important. In the creeds of the early church catholicity is confessed as characteristic for the church. This means that the church cannot exist without catholicity. What does this qualification mean? In this study the author listens to the understanding of the concept of catholicity in the theology of Ignatius, Cyprian, Cyril, Augustine and Vincent. In the second part of the book some representatives of the reformed tradition are analyzed, namely John Calvin, James Ussher, John Owen, Herman Bavinck and Gerrit Berkouwer. This analysis leads to a comparison between the early church and the reformed tradition. Listening to theologians from the early church and the reformed tradition, Van Vlastuin presents an up-to-date concept of the catholicity of the church which clarifies among others that the visibility of the church belongs to the essence of Christ's body, that practicing the catholicity of the church is necessary against denominationalism and party formation, that loss of catholicity leads to spiritual and theological impoverishment, that the understanding of catholicity implies also orthodoxy with consequences for the interpretation of 'semper reformanda' and that the consciousness of catholicity is related to the citizenship of two worlds.