Knowing that of the Lord ye shall receive the reward of the inheritance: for ye serve the Lord Christ.
since you know that you will receive an inheritance from the Lord as a reward. It is the Lord Christ you are serving.
εἰδότες ὅτι ἀπὸ Κυρίου λήψεσθε τὴν ἀνταπόδοσιν τῆς κληρονομίας· τῷ γὰρ Κυρίῳ Χριστῷ δουλεύετε.
eidotes oti apo Kyrioy lēpsesthe tēn antapodosin tēs klēronomias· tō gar Kyriō Christō doyleyete.