The Saint Peter Apostle church of Andahuaylillas is a colonial church in the Cusco-Puno area of the Andes, dating to the late 16th or early 17th century and built primarily with adobe brick masonry. The church is characterized by a triumphal arch made with fired clay bricks and surmounted by a high tympanum of adobe masonry. The arch is stabilized by lateral walls of mixed stone-adobe construction. As part of the seismic assessment of the building, this work focuses on the evaluation of the triumphal arch subjected to in-plane lateral accelerations by pushover analysis based on nonlinear FE explicit formulation. Detailed 2- and 3D FE models are constructed from measurements taken on location and observed material distributions of adobe, fired bricks, and stone. The FE models are analyzed in Abaqus/CAE Explicit under gravitational loading and monotonically increasing lateral accelerations, simulating quasi-static conditions until collapse. The concrete damaged plasticity formulation is adopted to represent masonry materials with nonlinear tensile and compressive behavior based on the Lourenço’s model. Local and global collapse conditions are identified from the energy and reactions curves. The effects of lateral walls and their material composition, nave walls, and the brick arch on the lateral capacity the triumphal arch are systematically evaluated.