Book excerpt: John Zizioulas is renowned for his controversial reflection on the ontological freedom as the cause and cipher of God’s being, which also has important implications for anthropology, ecclesiology and ecumenical dialogue. This view is bound up with a personalist conception of the Trinity, recognised in the teaching of the Greek Church Fathers, in which the person represents the primary ontological category. In particular, Zizioulas shows how, by virtue of the Father, personhood coincides with absolute freedom. In The Father’s Eternal Freedom, Dario Chiapetti explores this ontology. Taking into account Zizioulas' epistemological principles, his patristic reading and his theological development, the author systematically presents Zizioulas' thesis, verifying its conformity to dogma and its internal coherence. Chiapetti analyses how Zizioulas' proposal brings back to the centre of systematic theology the teaching of the Greek Fathers, especially the Cappadocians, and the apophatic horizon of dogmatic reflection. Such reflection pushes the discourse on God to its maximum degree, identifying and bringing out, rather than resolving or attenuating, the aporetic terms that structure it.