A.T. Robertson Commentary Titus 1:15

"To the pure all things are pure: but to them that are defiled and unbelieving nothing is pure; but both their mind and their conscience are defiled." — Titus 1:15 (ASV)

To them that are defiled (τοις μεμιαμμενοις). Perfect passive articular participle of μιαινω, old verb, to dye with another colour, to stain, in N.T. only here, Jude 1:8; Hebrews 12:15. See μεμιαντα (perf. pass. indic.) in this verse. Μολυνω (1 Corinthians 8:7) is to smear.

Unbelieving (απιστοις). As in 1 Corinthians 7:12f.; 1 Timothy 5:8. The principle or proverb just quoted appears also in 1 Corinthians 6:12; 10:23; Romans 14:20. For the defilement of mind (νους) and conscience (συνειδησις) in both Gentile and Jew by sin, see Ro 1:18-2:29.