Be­hind the Vat­i­can’s me­dieval walls, 133 car­di­nals gath­ered for Mass on Wednes­day morn­ing in prepa­ra­tion for the start of a con­clave to elect a suc­ces­sor to Pope Fran­cis.

At 4:30 p.m. (1430 GMT; 10:30 a.m. EDT), the car­di­nals will walk in­to the Sis­tine Chapel, chant­i­ng the med­i­ta­tive “Litany of the Saints” and the Latin hymn “Veni Cre­ator,” im­plor­ing the saints and the Holy Spir­it to help them pick a pope.

Once there, they pledge to main­tain se­cre­cy about what’s about to tran­spire and to not al­low any in­ter­fer­ence from out­siders to in­flu­ence their vot­ing.

Stand­ing be­fore Michelan­ge­lo’s vi­sion of heav­en and hell in “The Last Judg­ment,” each car­di­nal places his hand on the Gospel and swears to car­ry out that du­ty. The awe­some­ness of the chapel’s fres­coes, and Michelan­ge­lo’s in par­tic­u­lar, is meant to re­mind the car­di­nals of the weighty re­spon­si­bil­i­ty they bear.

Af­ter the car­di­nals take their oaths, a se­nior car­di­nal de­liv­ers a med­i­ta­tion. The mas­ter of pa­pal litur­gi­cal cer­e­monies, Arch­bish­op Diego Rav­el­li, calls out “Ex­tra omnes,” Latin for “all out.” Any­one not el­i­gi­ble to vote then leaves and the chapel doors close, al­low­ing the work to be­gin.