But what does the text say?

Contrary to Schreiner, Witherington, Moo, and others who maintain this guaranteed future reality, there is no indication within the context of Romans 8:30 that Paul writes from this future standpoint. Assurance alone is not a strong rationale for assuming Paul is writing about believers’ guarantee of glorification from a future perspective. I suggest that scholars take this view because their presupposed definition of δόξα [doxa] requires it: God’s people have clearly not yet been brought fully into the divine glory; therefore, the only explanation of the aorist is that, because it is so assured in the future, it can be spoken of as if it were a reality already in the present. This anticipatory interpretation, however, is unsupported.—Conformed to the Image of His Son, 236