When the Shroud of Turin goes on display Sunday for the first time in five years, it will revive a long-running debate as to whether it is a medieval fabrication or—as Catholic devotees have believed for centuries—the burial cloth of Jesus Christ.

But that debate raises the larger question of why Catholics venerate the shroud—and countless other relics.

The fervor surrounding each display of the shroud testifies to the power such relics command in the church. More than a million people have already reserved a free ticket for an up-close view of the shroud, which will be displayed until June 24. Visitors will file past the shroud for 12 hours a day, and about a fifth of the available dates are sold out.

The 14½-foot-long piece of linen, which bears a front-and-back image of a dead man’s body, is owned by the pope but safeguarded by the archdiocese of Turin. The relic traditionally has gone on view only once or twice a century, but it has been displayed five times since 1933. The archdiocese held an extraordinary showing in 2000 for the millennial Jubilee and another in 2010, in part to draw visitors to the city amid the economic downturn.

This time, the archdiocese is displaying the shroud to commemorate the 200th anniversary of the birth of St. John Bosco, a Turin priest who founded the Salesian order in the 19th century to help disadvantaged young people. Pope Francis, whose family hails from the area outside Turin, will pray in front of the shroud on June 21.