ISB ~ Ephesians 5 v32

mystery:Marriage is an earthly image of the heavenly union between Christ and the Church. This spiritual symbolism was hidden from the beginning in the marital covenant and is now manifest in the New Covenant. Saint Jerome rendered the Greek word for “mystery” as sacramentum (sacrament) in the original Latin Vulgate (CCC 774-76).

See word study:Mystery at Eph 3:3.