This article explores Austin Farrer's contribution to trinitarian theology, arguing that he grounds understanding of the doctrine of the Trinity in the life of prayer. While Farrer nowhere offers a systematic presentation of the believer's experience of the Trinity, an investigation of his writings, particularly his sermons and devotional works, reveals that it is precisely in prayer that he thinks the force of the doctrine is revealed to the believer. Beginning with Farrer's ‘empirical principle’, the idea that to know anything one must exercise one's relation to it, the article attempts to show how the act of praying constitutes a living out of the doctrine of the Trinity. Living in the Son entails an adoption of an attitude of sonship towards the Father, which Farrer describes most succinctly as an ‘active openness of heart’. This filial attitude, which Christ expressed humanly throughout his life, is adopted by believers through the Holy Spirit who, according to Farrer, is not an object of direct experience. In this, his trinitarian understanding of prayer differs from Sarah Coakley's, whose reflection on this topic serves as a point of comparison at various places throughout the article. Through Coakley's work, the trinitarian nature of prayer has become a theme in contemporary theology. Thus, this article is aimed at more than simply illuminating a somewhat neglected aspect of Farrer's thought; it is also an attempt to contribute to an ongoing, constructive conversation about the Trinity in the life of faith.