Athanasius (ad Episcopos Ægypti, § 13), referring to 1 John v. 20, This is the true (ἁληθινός) God, writes:  “But these men (the Arians), as if in contradiction to this, allege that Christ is not the true God, but that He is only called God, as are other creatures, in regard of His participation in the Divine nature.”  Again (c. Arian. iii. 9), “He gave us to know that of the true Father He is the true Offspring (ἀληθινὸν γέννημα).