Marius the Epicurean: his sensations and ideas is a historical and philosophical novel by Walter Pater (his only completed full-length fiction), written between 1881 and 1884, published in 1885 and set in 161–177 AD, in the Rome of the Antonines.

It explores the intellectual development of its protagonist, a young Roman of integrity, in his pursuit of a congenial religion or philosophy at a time of change and uncertainty that Pater likened to his own era.

Marius, a sensitive only child of a patrician family, growing up near Luna in rural Etruria, is impressed by the traditions and rituals of the ancestral religion of the Lares, by his natural surroundings, and by a boyhood visit to a sanctuary of Aesculapius.

As a youth he is befriended by and falls under the influence of a brilliant, hedonistic older boy, Flavianus, who awakens in him a love of literature (the two read with delight the story of Cupid and Psyche in Apuleius, and Pater in due course makes Flavian, who is "an ardent student of words, of the literary art",[2] the author of the Pervigilium Veneris).

Later he is taken by Cornelius to a household in the Campagna centred on a charismatic young widow, Cecilia, where prevails an atmosphere of peace and love, gradually revealing itself as a new religion with liturgy and rituals that appeal aesthetically and emotionally to Marius.

The sense of purposeful community there, set against the persecution of Christians by the authorities and the competing philosophical systems in Rome, contributes to Marius' mood of isolation and emotional failure.

Marius the Epicurean explores a theme central to Pater's thinking and already examined in his earlier Imaginary Portrait 'The Child in the House' (1878): the importance to the adult personality of formative childhood experiences.

He remains essentially Epicurean: His epiphany in the Sabine Hills, where he sensed a "divine companion" and the existence of a Platonic "Eternal Reason" or Cosmic Mind, is not a prelude to religious faith, though it continues to comfort him.

Some readers take the novel at face value, as a conversion narrative;[18] others may feel that Pater makes it hard for them to believe that Marius, with his acute, probing, restless mind, would have embraced Christian doctrines if he had examined them.

In an early review in Macmillan's Magazine the novelist Mary Ward praised "the great psychological interest" of the book, but identified as a weakness its tendency to depict Christianity from an aesthetic viewpoint, rather than presenting it as life's ultimate truth and reality.

[25] Pater's interspersing of narrative with classical and historical texts – borrowings acknowledged and unacknowledged, translations and adaptations – makes Marius the Epicurean an early example of a novel enriched by intertextuality.

"[30] Though unfinished, Gaston throws light on Pater's intentions in Marius, as well as further developing his experimental technique: "Dramatic action is filtered by memory, ideas, and multiple perspectives," writes Monsman, "dissolved so radically that the fictional protagonist seems almost to be reading about the age in which he lived.