quote: What is an heir and what does it mean to be joint heirs?
The first thing heirs are in this chapter are possessors of God as a Father in life. Of course they had to be forgiven and justified in order to have God as their Father.
The Holy Spirit bears witness deep within in their regenerated spirit that God is now their dear Father. They inherit God is "Abba Father" - a sweet personal and intimate possession.
They possess His life. This elementary relationship with God restricts His chirldren and limits His children in a saving way; a transforming and conforming way. In the process of them learning to live by a new life relationship they mature from children to sons.
Sons are qualified to be co-heirs with the Firstborn Son Jesus. Suffering helps to conform them into maturity. This is why Paul labors to teach the believers to have a positive attitude towards these sufferings. They work to prepare the coming glory to be revealed in them:
"For I consider that the sufferings of this present time are not worthy to be compared with the coming glory to be revealed upon us." (v.18)
So the inheritors of God should believe that God loves them and is able to cause all things to work together for the fulfillment of His eternal purpose.
"And we know that all things work together for good to those who love God, to those who are called according to His purpose. " (v.28)
What does suffering DO for the believers? It causes them to HAVE to live by another life which is able to pass through anything. That is Christ Himself indwelling them (vs. 9-11).
If we allow this suffering to drive us deeper into living by and in Jesus Christ, we will be conformed to His image and prepared to be co-heirs with Him at His return.
The inheritance of Christ is a man who expresses God. God desires that all be perfected to share in the expression of God in humanity along with His Son. It is a destiny that has been pre- marked out. And in the eyes of God, it has taken place already. So He speaks of glorified in verse 30 in the past tense.
It is sure and certain to take place that believers are conformed to the image of Christ the Firstborn Son of "many brothers".
"Because those whom He foreknew, He also predestinated to be conformed to the image of His Son, that He might be the Firstborn among many brothers;
And those whom He predestinated, these He also called; and those whom He called, these He also justified; and those whom He justified, these He also glorified." (vs.29,30)