20. κατάλυε τὸ ἔργον τοῦ θεοῦ. The οἰκ., the duty of Christian to Christian, is GOD’s own work; cf. 1Co 3:9; Act 20:32. καταλ. is suggested by the metaphor of building; cf. Mar 15:29; Gal 2:18; 2Co 13:10.
πάντα μὲν καθαρά. The admission of Rom 14:14 is repeated, to bring out more explicitly the harm which may be done by insisting on rights; 1Co 10:23; 1Co 8:9.
ἀλλὰ κακὸν, sc. your use of this principle, τὸ τῇ ἐξουσίᾳ χρῆσθαι. The assumption, as throughout, is that the weak brother may be led to act against his conscience by the example of the strong.
διὰ προσκόμματος. Under conditions which will make him fall. διὰ w. gen. expresses the conditions of an action; cf. Rom 2:27, Rom 8:25; 2Co 2:4; Blass, p. 132 f.