On July 16, 1919, the Holy Office issued a decree declaring the teachings of the Theosophical Society incompatible with Catholic doctrine and forbidding the faithful from participating in its activities. Drawing on previously unexamined Vatican archival materials, this article reconstructs the five-year investigation that led to the decree and situates it within a longer institutional experience through which the Church confronted and regulated emerging spiritualities in an era of intense secularization. By analyzing the reports produced by different consultors, the article shows how Theosophy was interpreted through inherited classificatory frameworks shaped by earlier encounters with animal magnetism, Spiritualism, and occultism, and then refracted through the anti-modernist polemics of the early twentieth century. More broadly, the case sheds light on how the Vatican attempted to govern new forms of transnational religiosity that challenged established modes of ecclesiastical authority.