T. S. Eliot is famous for his use of allusions. His best-known poem, "The Waste Land," incorporates quotations and references spanning thousands of years of literary history. Though Four Quartets isn't quite as allusion-heavy, it's still dense with references.
First, the Quartets are prefaced by two epigraphs from the ancient Greek philosopher Heraclitus (or Herakleitos):
The law of things is a law of Reason Universal, but most men live as though they had a wisdom of their own. (J. M. Mitchell, trans.)
The way up and the way down are one and the same. (Guy Davenport, trans.)
The philosophy of Heraclitus deals extensively with the flow of time (his best-known statement is that "No man ever steps into the same river twice"). It also focuses on the unity of opposites, a concept illustrated by the second epigraph above (and paraphrased in lines 125-126 here: "This is the one way, and the other / Is the same"). Meanwhile, the first epigraph evokes the kind of universal wisdom the Quartets seek to impart as a balm for the ills of modern life. The poem's statements about time (e.g., lines 1-5) incorporate some of Heraclitus's thinking on this subject, along with St. Augustine's reflections on time in the Confessions (c. 400 CE).
Eliot's references to "unheard music" (line 29) and the "stillness" of the "Chinese jar" (lines 145-146) echo John Keats's "Ode on a Grecian Urn" (1819), a famous poem about an ancient urn (ornamental jar) whose "quietness" paradoxically conjures up "melodies [...] unheard." In both poems, the haunted and haunting silence is related to the effects of both memory and poetry.
The word "lotos" in line 38 carries several possible literary/religious echoes. This fruit of the lotos/lotus tree appears in Homer's Odyssey (Book IX), as well as in Alfred, Lord Tennyson's Homer-referencing poem "The Lotos-Eaters" (1832). In both works, the lotos is a pleasant drug that causes users to forget all worldly attachments and cares. The lotus flower is also an important symbol in (for example) the Hindu and Buddhist traditions, where it's associated with divine perfection and purity. Here, the mirage-like "lotos," rising in the garden "pool," might symbolize any or all of these things; broadly, it's an image of purity, holiness, and contentment.
Another likely allusion comes in lines 49-50: "Garlic and sapphires in the mud / Clot[ting] the bedded axle-tree." Some critics believe this odd image alludes to the 19th-century French poet Stéphane Mallarmé’s "M’introduire dans ton histoire," which describes "Tonnerre et rubis aux moyeux" (thunder and rubies at the wheel-hubs). Mallarmé’s poem goes on to describe "kingdoms shattered and dispersed" and the metaphorical "chariot [wheel] of evening," so Eliot's poem may be adapting this apocalyptic image to symbolize modern cultural brokenness.
Christian references sprinkle the poem as well, reflecting Eliot's personal faith. Lines 158-161 ("The Word in the desert / Is most attacked by voices of temptation") refer to the biblical Temptation of Christ (see Matthew 4:1-11). "The figure of the ten stairs" (line 163) refers to an image from "Dark Night of the Soul," by the 16th-century Christian mystic St. John of the Cross, which envisions ten steps on the "ladder of divine love."