well, here are a couple of references:
Romans 8
14 For all who are led by the Spirit of God are sons of God.
15 For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!”
16 The Spirit himself bears witness with our spirit that we are children of God,
17 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.
what does this mean exactly? to be an heir we must inherit something.
Ephesians 3:6 This mystery is that the Gentiles are fellow heirs,
members of the same body, and partakers of the promise in Christ Jesus through the gospel.
fellow heirs? (with who?), Not "the body" but "members of the same body" (with who else?)
Galatians 3:29 And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise.
We are grafted in and we are in Covenant with God.