This paper investigates a possible theological influence of Irenaeus of Lyons upon the Creed formulated at the Council of Nicaea. The most obvious starting point for such a study is with the Irenaean rule of faith (regula fidei). The relationship of this second-century expression of faith to the Creed of 325 is a fascinating one. Indeed, Montgomery Hitchcock pointed out more than a century ago how “remarkable” it is that the Irenaean writings anticipated the Creed, which was so “similar in sublime phrase and logical order” (Hitchcock, 1914, 41). Although this paper will introduce the relationship of Nicaea to Irenaeus vis-a-vis the regula fidei, the primary area of investigation will probe the concepts of “substance” and “light.” A unique feature of the Creed was the description of Jesus as the one who is “from the substance of the Father, God from God, light from light.” The language of “substance” (homoousios / substantia) and the description of the Son of God as being “light from light” are not found in the New Testament, but were expressions used by Irenaeus nearly a century-and-a-half prior to the great council. This essay will examine the use of these key terms in Against Heresies IV, chapters four through fourteen, demonstrating the notable ways Irenaeus’ thought anticipated the Nicene formulation. In so doing, I will demonstrate the second-century’s important, generally overlooked contribution to this key development in early Christian orthodoxy.