Nyssen’s arguments in In Illud: Tunc et Ipse Filius entirely derive from Origen (probably also passing through Marcellus of Ancyra and Eusebius). Origen’s influence, theoretical and exegetical, is evident in every passage, from the argumentative pillars down to the tiniest details of exegesis. Gregory’s close dependence on Origen in his anti-subordinationism, within his polemic against ‘Arianism,’ confirms that Origen was not the forerunner of ‘Arianism,’ as he was depicted in the Origenistic controversy and is often still regarded to be, but the main inspirer of the Cappadocians, especially Nyssen, in what became Trinitarian orthodoxy. Origen inspired Marcellus, who was anti-Arian, Eusebius, who in fact was no ‘Arian,’ Athanasius, the champion of anti-Arianism, and the Cappadocians. I argue extensively that Origen’s Trinitarian heritage is found, not in Arianism, but in Nyssen, Athanasius, Eusebius, and the Nicene-Constantinopolitan line, on the basis of a painstaking analysis of his works (always with attention to their reliability in relation to Greek original, translations, and fragments) and of Pamphilus, Eusebius, Athanasius, and other revealing testimonies, pagan and Christian. The origin of the µοοσιος formula is also investigated in this connection. Further interesting insights will emerge concerning Eusebius and his first report of what exactly happened at Nicaea.