Ephesians 5:32

τὸ μυοτήριον τοῦτο μέγα ἐστίν. Cf. Hort Chr. Eccl. p. 151. ‘If we are to interpret ‘mystery’ in the difficult 32nd verse, as apparently we ought to do, by St Paul’s usage, i.e. take it as a Divine age-long secret only now at last disclosed, he wished to say that the meaning of that primary institution of human society, though proclaimed in dark words at the beginning of history, could not be truly known till its heavenly archetype was revealed, even the relation of Christ and the Ecclesia.”

μέγα ‘important’ is applied to μυστήριον also in 1 Timothy 3:16.

λέγω εἰς, Hebrews 7:14. ‘I speak with reference to.’