A rhetorical expression not to be taken literally.  Some of the enormities committed under Valens, e.g. the alleged massacre of the Orthodox delegates off Bithynia in 370 (Soz. vi. 14, Theod. iv. 21), would stand out even when matched with the cruelties perpetrated under Nero and Diocletian, if the evidence for them were satisfactory.  cf. Milman, Hist. Christ. iii. 45.  The main difference between the earlier persecutions, conventionally reckoned as ten, and the persecution of the Catholics by Valens, seems to be this, that while the former were a putting in force of the law against a religio non licita, the latter was but the occasional result of the personal spite and partizanship of the imperial heretic and his courtiers.  Valens would feel bitterly towards a Catholic who thwarted him.  Basil could under Diocletian hardly have died in his bed as archbishop of Cæsarea.