Irenaeus of Lyon was truly a theologian of catholicity. The second-century bishop embraced an aesthetic of "symphonic" unity in diversity (AH 4.14.2) in multiple registers throughout his major extant work, Adversus haereses (AH). In this paper, I look more closely at Irenaeus' musical illustration of this aesthetic - his analogy of the musician and the lyre (AH 2.25.2). Irenaeus describes a musician drawing together the disparate tones of the musical instrument into a harmonious melody. While the pitches played on the lyre, when viewed individually, are "contrary and unharmonious" (contraria et non conuenientia) to each other, the musician—i.e., God the Creator—arranges them so that they are "well-adapted and symphonious" (bene aptata et bene consonantia). I consider this passage from the perspective of musical composition and performance, exploring how Irenaeus' seemingly counterintuitive claim that unity and beauty arise not despite difference and dissonance, but precisely because of them, is rendered aesthetically reasonable in a musical context, within which melodic inventiveness and beauty are not guaranteed, but rather require the particular skill and sensitivity of the master musician. This form of order is not hierarchical or linear, but requires a dynamic, responsive, organic sense of beauty-a "cosmic" ordering of creation that reflects the Divine Musician-the Father, with whom nothing is "incongruous" (incongruens), and the Son, who works out the things of the Father "at the right time, in the right order and sequence" (sicut congruum et consequens est, apto tempore) (AH 3.16.7). Irenaeus' engagement with music reveals catholicity as a mode of creaturely unity fundamentally based not on harmony of creaturely characteristics (uniformity), nor resulting from direct creaturely agency toward a single goal (unification), but depending entirely on unity of creaturely relation to God, who is the ground and agent of true unity.