As one of the largest real estate owners globally, the Catholic Church acknowledges the urgency of decarbonizing its buildings to mitigate climate change. This paper examines potential actions needed to reduce greenhouse gas emissions from a Catholic Church building in Los Altos, CA, accounting for operational and embodied carbon. Possible interventions for decarbonizing the building were identified using a prototype protocol developed at Stanford University. Since the building was constructed without a cooling system, the paper considers the effects of adding cooling to the building in addition to the decarbonization actions. EnergyPlus, a whole-building simulation program, was used to model the building energy consumption. The annual greenhouse gases emitted by the building were calculated according to the national standards and projected from 2025 through 2045. The study also considers the impact on emitted carbon if the building purchases electricity from suppliers with different carbon intensities. The paper shows that when electrifying building systems, purchasing off-site low-carbon-intensive electricity is critical to building decarbonization and may lead to a potential reduction of 64–91% in carbon emissions. Furthermore, the study shows that when considering sustainable operating costs, a significant reduction of the building’s greenhouse gas emissions is still possible – potentially up to 94–109% – with operating costs ranging from an increase of 26% to a decrease of 80% compared to the baseline. Based on the findings of this study, purchasing off-site low-carbon-intensive electricity is the first crucial action the Catholic Church should implement to decarbonize its buildings.