User blog comment:Jopjopjop/Volume 20 Spoilers/@comment-178.128.187.173-20150830110103/@comment-26865730-20150907194744

"Deification" has not been fully explained so how far along it puts Issei on the path to "God-hood" is unknown. The term itself implies some change to the divine.

As for what they mean, I think Ishibumi-san has made a distinction between just beng super-strong and being "Divine". I'm thinking of the replica mjolner (sp) Issei used against Loki. Issei could not use it until he received the divine spark from the Seretie of Oppai. Also, Ophis is Divine even though no longer the strongest. eg.: GR=True Dragon, Ophis=God Dragon. The "True" & "God" modifiers here are refering to their nature and not their strength. Otherwise Crom Crauch might well claim to be a "God Dragon" and I don't think Ishibumi-san has even implied that status for C.C.