“The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.” – Romans 8:16-17 (ESV)
“To say that believers are [only] heirs of God, however, leaves out a major motif in Paul’s thinking, for believers are also “fellow heirs with Christ.” This statement does not function as a corrective to the previous one but elucidates the means by which believers are heirs of God. The inheritance becomes a reality through union with Christ, the true seed of Abraham (Gal. 3:16). Those who are united with Christ share in the inheritance that he has gained for them.”
– Thomas Schriener, Romans (Grand Rapids: Baker, 1998), 428.
“Paul is… reminding us that Christians inherit the blessings of God’s kingdom only through, and in, Christ. We, “the sons of God,” are such by virtue of our belonging to the Son of God; and we are heirs of God only by virtue of our union with the one who is the heir of all God’s promises (see Mark 12:1-2; Gal. 3:18-19; Heb. 1:2).”
– Douglas Moo, Romans (Grand Rapids: Eerdmans, 1996), 505.