Matthew 13:11

τὰ μυστήρια τῆς βασιλείας τῶν οὐρανῶν. Secrets known only to the initiated—the inner teaching of the gospel. St Paul regards as ‘mysteries,’ the spread of the gospel to the Gentiles, Ephesians 3:3-4; Ephesians 3:9; the doctrine of the resurrection, 1 Corinthians 15:51, the conversion of the Jews, Romans 11:25; the relation of Christ to His Church; Ephesians 5:32.

To the Greek, μυστήρια would recall the associations of Eleusis and Samothrace, and so necessarily bring a part of the mystic thought into Christianity; only, however, to contrast the true Christian mysticism, which is open to all (νῦν δὲ ἐφανερώθη τοῖς ἁγίοις αὐτοῦ, Colossians 1:27), with the secresy and exclusiveness of the pagan mysteries. Bp. Lightfoot on Colossians 1:21-28. The derivation is from μύειν, ‘to close the lips.’ The initiated are called μεμυημένοι or τέλειοι (fully instructed); the use of the latter word may be applied to the same conception in 1 Corinthians 2:6, σοφίαν λαλοῦμεν ἐν τοῖς τελείοις … θεοῦ σοφίαν ἐν μυστηρίῳ κεκρυμμένην. See also Philippians 3:15; Hebrews 5:14.