In 1929 Trethowan was received into the Roman Catholic Church and took a job as a schoolmaster at the Oratory School, London, later transferring to Ampleforth.

In 61 years of unswervingly faithful monastic life, he lived what he taught: that it is possible for the human mind to be aware of God.

[3] For over twenty years, he was in dialogue with Eric Mascall, with whose work Louis Bouyer draws comparisons, calling Trethowan "a born Augustinian, but of exceptional intellectual acuity".

[4] Trethowan’s main contribution to the philosophy of religion is the argument that human awareness is intelligent and that the transcendent (divine, absolute, infinite) is implicit in it.

The concepts at the core of his thinking (preconceptual, direct but mediated awareness) are illustrated in the phenomenological example he gave in Mysticism and Theology: A fly settles on my nose while I am asleep.

[10]But Trethowan argued no less tirelessly against efforts made in various forms of Thomism (as he saw it) to ground faith in reason rather than experience.

Bellenger noted that Trethowan was "particularly influential in introducing French Catholic philosophy to an English audience and in breaking the stranglehold of Thomism.

Like Blondel, Trethowan argued that left to its own resources philosophy can only reach an impasse, the only way out of which is to accept the notion of the transcendent, which opens the mind to the possibility of faith, the thesis of theology.

This approach to the concept of the supernatural is close to that of Henri de Lubac in his Surnaturel and to the early Karl Rahner[13] As a monastic thinker Illtyd Trethowan moved easily between the fields of theology, literature, and philosophy.

His first book on the theology of the Eucharist, as well as Christ in the Liturgy, anticipated issues that were to loom large in the Roman Catholic liturgical movement and at Vatican II.

A reviewer of this book wrote that it "awakens hopes of a Christocentric synthesis the formulation of which would call forth his best from one who is by vocation a liturgist, is a theologian by trade and a philosopher by inclination.

A reviewer summarized the work of Trethowan by noting: As regards Illtyd and his writing, I have long had the sense: Here is somebody who has been there with God, who is close to the heart of the matter, and who is now seeking, in ways whose conceptual specifics I may variously judge more, or less, well-attuned, to assist others to share this also.