Ambrose of Milan quotes in De Fide III, 15, 125 a passage from a letter that was read out at the Council of Nicaea (325) and which he attributes to the important Arian bishop Eusebius of Nicomedia. In this quotation, the term ‘homousion’ appeared as a concept contested by the Arians and, precisely because of this rejection, according to Ambrose, the word was included in the Nicene creed. However, until now, this letter of Eusebius of Nicomedia has not been identified, since the key term homoousios does not ap-pear in his writings. In this paper, it is proposed, by means of a direct and indirect textual demonstration, that the letter alluded to by Ambrose is the Arian creed whose íncipit is Ἡ πίστις ἡμῶν (Opitz’ Urkunde 6), since this document contains the term ‘ὁμοούσιον’ literally in several grammatical cases. It is argued that, in a con-text of insufficient linguistic fixation of the faith, terminology was an essential question, and therefore the leaders of the Council of Nicaea must have arrived at the conclusion that the condemnation of Arianism had to be carried out with linguistic resources drawn from Arian documents, in order to defeat the heresy with its own weapons. Ἡ πίστις ἡμῶν homoousios was the most important of these weapons and it was also very directly linked to Eusebius of Caesarea’s creed, which was presented at Nicaea as an appeal against his conviction in the Council of Antioch.