Library Eusebius of Nicomedia Letters and Fragments
Fragment on Homoousios (via Ambrose)
Quoted by Ambrose, Exposition of the Christian Faith (De Fide) 3.15.125 (NPNF). Ambrose says this line from Eusebius of Nicomedia was read at Nicaea and helped push the Fathers to insert homoousios into the creed.
What other reason can there be for their unwillingness to have the Son spoken of as homoousios, of the same substance, with the Father, but that they are unwilling to confess Him the true Son of God? This is betrayed in the letter of Eusebius of Nicomedia. “If,” writes he, “we say that the Son is true God and uncreate, then we are in the way to confess Him to be of one substance (homoousios) with the Father.” When this letter had been read before the Council assembled at Nicaea, the Fathers put this word in their exposition of the Faith, because they saw that it daunted their adversaries; in order that they might take the sword, which their opponents had drawn, to smite off the head of those opponents’ own blasphemous heresy.
English from Nicene and Post-Nicene Fathers, Series II: Theodoret, Ecclesiastical History (tr. Blomfield Jackson); Athanasius, De Synodis (tr. Archibald Robertson); Socrates, Ecclesiastical History (tr. A. C. Zenos); Ambrose, Exposition of the Christian Faith (NPNF). Public domain. Digital text via New Advent Church Fathers. In the letter to Paulinus, the clause κτιστὸν … τῇ οὐσίᾳ is aligned to the Greek where Jackson’s “in the same substance” is misleading.