The item is known as St. Anthony's relic. It is said to be more than 780 years old and had been with St. Anthony's Church since it was founded more than 100 years ago.
Usually kept out of the public eye because of its value, the church put it on display this week to commemorate the patron saint of the church.
The relic was encased in a golden container and on display inside a glass case near the altar. But when this morning's mass got under way, the relic was gone.
"It doesn't belong to you. It belongs to a lot more people," said Father Jose Magana, the pastor at the church near 6th Street and Olive Avenue. "It belongs to, not only our church, but it belongs to all of us."
Police said they have a person of interest in the case. They want to find a woman in a composite sketch they released. They said she attended several services Sunday and seemed fascinated by the relic.
"On a couple of occasions, they had to ask her to kind of step away from the cabinet because she was getting a little bit too close," said Lt. Paul Arcala of the Long Beach Police Department. "They were a little bit nervous about her behavior towards the cabinet."
Father Magana says he doesn't regret his decision to put the relic on display and is certain someone will return it.
The person of interest was described as a Hispanic woman, 35 to 45 years old, about 5 feet 2 inches tall, with dark brown hair.
Anyone with information was asked to call the Long Beach Police Department at (562) 570-5590.