This paper examines the ecumenical significance of John D. Zizioulas's eschatological ontology, approaching it from the standpoint of contemporary Catholic-Orthodox dialogue. It argues that Zizioulas's eschatological ontology of ecclesial existence offers not only a compelling vision of Christian unity but also a distinctive method for ecumenical engagement. By situating ecclesial identity primarily within the horizon of the eschaton, Zizioulas resists both juridical reductionism and historical absolutism. Proposing an understanding of the Church as a proleptic sign of the Kingdom of God instead of an institution with fixed identity in the past, Zizioulas reframes unity not as a product of institutional alignment or doctrinal harmonization, but as a participation in the anticipated fullness of the Kingdom of God, sacramentally realized in the eucharistic assembly. At the same time, the limits of Zizioulas' approach are critically assessed, especially regarding his tendency to limit the ecclesial and eschatological event to the eucharistic gathering. Ultimately, the paper proposes that Zizioulas's future-oriented hermeneutics can serve as a constructive bridge for ecumenical dialogue, recalibrating unity as an eschatological rather than protological reality, while sustaining concrete theological and ecclesial discernment in the present.