T. S. Eliot's last significant poems -- completed more than 20 years before his death -- are exquisite philosophical musings on the nature of time and history. Though they have a firm religious underpinning, shameless heathens like me still find them immensely beautiful. Eliot's "timeless moments" -- those instants of blinding epiphany and heightened existence that make the rest of life seem pathetically tame -- are common to all humans, as is the lament for the rarity of such experiences: "Ridiculous the waste sad time / Stretching before and after."
Be warned, however, that the Quartets are more uneven than most of Eliot's work. There are numerous passages of surprising blandness, as well as a few embarrassingly pompous lines ("I sometimes wonder if that is what Krishna meant" -- yeah, me too; all the time). In addition, the religion in the poems becomes progressively more explicit, which may or may not bother you. "Burnt Norton," the first quartet, was originally written to stand alone; it is the most continuously interesting and least Jesusy of the lot, as well as the shortest.
"Four Quartets" has sometimes been criticized for being more a product of reflection than emotion. While this is at least partly justified (I consider "The Waste Land" to be TSE's supreme masterpiece), 4Q remains a compelling, unforgettable poetic valediction by one of the greatest masters of the art.