The great philosopher Max Scheler of the University of Munich's convincing argument (himself a Catholic convert from a Lutheran/Jewish heritage) to Dietrich von Hildebrand for why he should convert to Catholicism
On the Legacy of Her Beloved Husband
Alice von Hildebrand EWTN
Dietrich's greatest debt to Scheler was that the latter's thought, still steeped in Catholicism, opened up for him the way to the Church by convincing him that she had both received and retained the fullness of revealed Truth.
This was to young Dietrich a revelation. Although born and raised in a Catholic country, he had never met a Catholic, let alone a convinced and practicing one. He was totally ignorant of the Church's doctrines. Two of his tutors (Ludwig Curtius and Alois Fischer) were fallen-away Catholics. He had been trained to enter churches as places rich in artistic beauty, but his parents were blind to their sacred character. Reverent as Dietrich was toward religion, he was nevertheless baffled by Scheler's assertion that the Roman Church was the one true Church of Christ, and he asked him to explain why he claimed this supremacy for the Catholic Church. With ease, Scheler brushed aside the dormant prejudices against the Church that the young man had picked up from both his tutors and his milieu. Then Dietrich's remarkable readiness to learn, his receptivity toward those who knew better than he, came to the fore. Scheler told him that the Church produced saints. "What is a saint?" the young man inquired. Once again, Scheler deployed the full scope of his genius. He applied all his gifts to sketching the essence of sanctity, and he illustrated his teaching by mentioning Francis of Assisi, describing his life in the most vivid terms. Scheler's arguments about holiness were so powerful that they convinced his young friend that the life of a Saint Francis of Assisi could not be explained by purely natural ethical categories; his holiness had to come from another source.
Alice von Hildebrand, The Soul of a Lion, San Francisco: Ignatius Press, 2000, 73-74.