Called as Missioner and Liturgist
But his studies never distracted him from what he would always call his "first love" -- the overseas missions. For twenty-five years, first as a doctoral student, then as a teacher of scripture at Holy Cross College, as mission procurator for the congregation of Holy Cross, and as founding superior of the community's Foreign Mission Seminary, Mathis devoted his abundant energies wholeheartedly to organizing every kind of support for the missions. In 1925, he also collaborated with Dr. Anna Dengel in founding the Catholic Medical Mission Society, better known as the Medical Mission Sisters, for whom he served as full-time chaplain from 1933-1938.
In 1936, while reading a copy of Pius Parsch's classic commentary The Church's Year of Grace, given to him by a respected colleague, Mathis underwent a conversion which surprised no one more than himself. Though he had always had a low opinion of liturgists, he found himself exclaiming: "This is the real stuff!" He later said of the experience: "I was converted to liturgy...and I do mean converted!" For the remaining 24 years of his life, especially after he was recalled from his beloved mission work in 1938, he re-directed his boundless energies to his second love: the promotion of the liturgy.