A.T. Robertson Commentary Romans 8:17

"and if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with [him], that we may be also glorified with [him]." — Romans 8:17 (ASV)

Joint-heirs with Christ (συνκληρονομο Χριστου). A late rare double compound, in Philo, an Ephesian inscription of the imperial period (Deissmann, Light from the Ancient East, p. 92), papyri of the Byzantine period. See 8:29 for this idea expanded. Paul is fond of compounds of συν, three in this verse (συνκληρονομοι, συνπασχωμεν, συνδοξασθωμεν). The last (first aorist passive subjunctive of συνδοξαζω with ινα (purpose), late and rare, here only in N.T.