Jamieson, Fausset and Brown on Romans 8:17

17. And if children, then heirs--"heirs also."

heirs of God--of our Father's kingdom.

and joint-heirs with Christ--as the "First-born among many brethren" (Ro 8:29), and as "Heir of all things" (He 1:2).

if so be that we suffer--"provided we be suffering with Him."

that we may be also glorified together--with Him. This necessity of conformity to Christ in suffering in order to participate in His glory, is taught alike by Christ Himself and by His apostles (Joh 12:24-26; Mt 16:24, 25; 2Ti 2:12).