User blog comment:Cookievalenzuela666/harem predictions......./@comment-79.45.189.189-20140329020354/@comment-16281343-20140329093600

The word "engagement" was used loosely because after those trials, Issei proved to the Gremories that he can represent them to the devils in a manner that wouldn't bring shame to their name.

Technically, they aren't engaged since the other partner (Issei) doesn't know anything about it. But loosely, you can say that they are since Rias and her family already accepted Issei, if a talk of marriage comes to play they wouldn't object to it; and Issei surely wants to marry Rias in the future.