“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)
“Paul asserts that believers have inherited the promise of Abraham (Gal. 3:14, 29), and this promise is an astounding one, for Abraham is heir of the world (Rom. 4:13…). Here he says something even more stunning: believers are “heirs of God” himself. The wording suggests not merely that believers are heirs of what God has promised but of God himself. The supreme benefit of the covenant with Abraham is not inheriting the land but having God as one’s God (Gen. 17:7).”
– Thomas Schreiner, Romans (Grand Rapids: Baker, 1998), 427-28.