The article analyses the theological foundations of the unity of the Church and the criteria of ecclesiality of Catholic communities. Its aim is to show that unity, understood from the perspective of the ecclesiology of communio, constitutes the basic criterion for assessing the place and role of communities within the Church. The first part discusses the Trinitarian, Christological-sacramental, and dynamic dimension of unity as both gift and task. It then presents the relationship between unity and communal pluralism. The second part interprets the criteria of ecclesiality of Catholic communities formulated by John Paul II in the apostolic exhortation Christifideles laici. The analysis leads to the conclusion that an authentic Catholic community realizes its identity through fidelity to the Church’s faith, sacramental life, and communion with its pastors.