Birgitta Trotzig (1929–2011), one of Sweden’s most influential modernist writers, characterizedherself as a religious author. Investigating her autobiographical comments in interviews,essays, radio programmes etc., the article attempts to reconstruct her own fundamental theology.Swedish literary scholars usually define Trotzig’s religiosity as a kind of mysticism. Thisarticle argues, however, that her attitude towards faith was remarkably rational. As reflectedin the analysed material, her personal creed evolved following a sequence of five intellectualchoices. First, Trotzig dissociated herself from atheism and agnosticism, choosing to liveas a believer. Secondly, she rejected all religions worshipping non-personal beings and choseChristianity and its faith in a personal God. She then repudiated Protestantism and the Churchof Sweden and chose the old churches because of their rich theological and liturgical tradition.Fourth, she distanced herself from certain aspects of Orthodox spirituality and chose RomanCatholicism as her confession. In the fifth and final step, she renounced the official, conservativeCatholic dogmatism and chose the progressive, left-leaning Catholic movements, closelyconnected to the new Catholic French theology, la nouvelle théologie. The article thoroughlyelucidates the rational arguments formulated by Trotzig to justify her five religious choices.