A.T. Robertson Commentary Romans 14:14

"I know, and am persuaded in the Lord Jesus, that nothing is unclean of itself: save that to him who accounteth anything to be unclean, to him it is unclean." — Romans 14:14 (ASV)

I know and am persuaded in the Lord Jesus (οιδα κα πεπεισμα εν κυριω Ιησου). He knows it and stands persuaded (perfect passive indicative of πειθω, to persuade), but in the sphere of the Lord Jesus (cf. 9:1), not by mere rational processes.

Unclean of itself (καινον δι' εαυτου). So Paul takes his stand with the "strong" as in 1 Corinthians 8:4f. , but he is not a libertine. Paul's liberty as to food is regulated by his life in the Lord. For this use of κοινος, not as common to all (Acts 2:44; Acts 4:32), but unhallowed, impure, see on Mr 7:2,5; Acts 10:14,28. God made all things for their own uses.

Save that (ε μη). The exception lies not in the nature of the food (δι' εαυτου), but in the man's view of it (to him, εκεινω, dative case).