Which consists or is in the inheritance. Compare the similar construction, Col 1:12. See Mat 21:35-38, where the δοῦλος bond-servant and the κληρονόμος heir are contrasted; and Rom 8:15-17; Gal 4:1-7.
For ye serve (γὰρ δουλεύετε)
Omit for. Some take the verb as imperative, serve ye; but the indicative is better as explaining from the Lord.