User blog comment:B214/Volume 22 spoilers/@comment-175.110.173.31-20160830041025/@comment-24.113.252.213-20160831054138

Ophis and Vali said that Issei's DxD G (or "Psuedo" Dragon Deification), now has room to grow and develope along with Issei.

I think Issei mastering the Cardinal Crimson Promotion might be the (or "a") key to mastering the DxD G.