In 1244 he left this retreat, and went with two companions to a similar cave on the Mountain of Maiella in the Abruzzi region of central Italy, where he lived as strictly as possible according to the example of St. John the Baptist. There he founded the order subsequently named after him, the Celestines. Pietro gave them a rule formulated in accordance with his own practices. In 1264 the new institution was approved by Urban IV. In 1274 the than pope Gregory reapprove his order, formally making it a branch of the Benedictines and following the rule of Saint Benedict, but adding to it additional severities and privations. The Celestines saw a rapid spread and Pietro lived to see himself "Superior-General" to thirty-six monasteries and more than six hundred monks. As Pietro had seen his new order thus consolidated he gave up the government of it to a certain Robert, and retired once again to a still more remote solitude to give himself up more entirely to solitary penance and prayer.
|this is the letter sent by the Cardinal electors of the 1292-1294 Conclave|
to the pious hermit Pietro di Murrone asking him to become Pope.
With no political experience, Celestine proved to be an especially weak and incompetent pope. Realizing his lack of authority, he consulted with Cardinal Benedetto Caetani (his eventual successor) about the possibility of resignation. This resulted in one final decree declaring the right of resignation, which he promptly exercised in January 1295, after only five months and eight days in office. In the formal instrument of renunciation, he recited as the causes moving him to the step: "The desire for humility, for a purer life, for a stainless conscience, the deficiencies of his own physical strength, his ignorance, the perverseness of the people, his longing for the tranquility of his former life". Having divested himself of every outward symbol of papal dignity, he retired to his old solitude.
However, the former Celestine, Pietro Angelerio, was not allowed to remain in solitude. Because certain parties had reason to worry that some might install Pietro as antipope, the next pope ordered Pietro to be brought to Rome. Pietro escaped and hid but eventually was captured and imprisoned in Ferentino, where he died after 10 months on 19 May 1296. Pietro was buried at Ferentino, but his body was subsequently removed to the Basilica Santa Maria di Collemaggio in Aquila.
A 1966 visit by Pope Paul VI to Celestine's place of death in Ferentino along with his speech in homage of Celestine prompted speculation that the pontiff was considering retirement. A similar visit by pope Benedict XVI on 28th April 2009, where he left on Celestine’s casket the long pallium with which he had been invested in 2005, was indeed followed by his abdication on February 28, 2013.
sources: wikipedia.org, historymedren.about.com, and theratzingerforum.yuku.com