SummaryChristian theologians can explain the Trinitarian faith today in dialogue with Islamic thinkers as “deepened monotheism”. Therefore it is important to widen the systematic-theological discourse in an ecumenical and transcultural perspective and to retrieve resources from Western and non-Western traditions of Trinitarian thought (I).In this paper I will first work out historically that the Trinitarian creed of Nicea and Constantinople was originally an ecumenical but non-Western creed (II). Afterwards, I investigate the philosophical-theological reflection on the Trinity by Anselm of Canterbury (...) (1033–1109) in the context of early interreligious encounters in the Latin West. Based on biblical, augustinian and Greek sources, he developed an approach to understand the mystery of the Trinity by rational arguments as “deepened monotheism” (III). Then I will proceed to explore the philosophical-theological dialogues on the Trinity from the Arabic philosopher and Syrian-orthodox theologian Yaḥyā ibn ‘Adī (893–974). Much earlier he developed rational arguments for the Triunity of God with reference to Aristotle. Thereby he answers to anti-trinitarian arguments from Islamic thinkers like al-Kindī and al-Warrāq. He intends that the Trinitarian faith of Christian minorities can thereby be understood and tolerated by Islamic thinkers as rationally founded “deepened monotheism” (IV).In the end I will evaluate what these classics from the Western and non-western traditions of Trinitarian thought contribute to explicate the doctrine of the Trinity today in a pluralistic religious context as “deepened monotheism” (V). (shrink)