User blog comment:B214/Question and Answers 2/@comment-92.84.65.178-20140204162005/@comment-1359322-20140204171210

It merely meant that his body still had some unexplored potential perhaps. I don't know. But i do know that the soul of a being is most important, take Ddraig and Albion for example, they are merely souls but the Sacred Gears still have their abilities. Issei's soul is that of a human, that's why Azazel is having Rias and the others continue taking the precaution steps. Regardless of how the new body is, Issei's soul is merely that of a human after all.