Irenaeus of Lyons wrote Demonstration of the Apostolic Preaching (Epideixis) to encourage his readers of the solidity of their faith, especially as this faith was connected to baptism under the threefold seal: Father, Son, and Holy Spirit. The threefold nature of the baptismal formula drives Irenaeus’ discussion in Epid. 3-7 and is the point with which he concludes the work, saying, ‘error, concerning the three heads of our seal, has caused much straying from the truth’ (Epid. 100). Irenaeus structures the intervening chapters to show how Christian baptism is both a testimony to and participation with the Triune God referenced in the baptismal formula. The lack of explicit structural markers within the body of the text has resulted in a neglect of the trinitarian character of its structure. This article explores the manner in which Irenaeus of Lyons’ Demonstration provides insight into his understanding of the nature and activity of the Triune God based on his conviction that Christian baptism is both a testimony to and a participation with that Triune God.