The Catechism in Questions & Answers

674. What happens when a church building is deconsecrated?

If a dedicated church is no longer being used for divine services, it is deconsecrated: in the last divine service conducted there, the purpose of the church building as a holy place of divine activity, as imparted in the dedication, is lifted. After its deconsecration, the former church is once again a regular building, which can be used for another purpose.