The “invisible mystery hidden in God” is indeed the Trinitarian aspect of God. God is not a single being. God is a community of persons, the Father, the Son and the Holy Spirit. Saint Augustine says that the Father loves the Son, the Son is loved by the Father and the Love between them is the Holy Spirit. The Christian family are called to make present this reality in the world.
Saint Paul also calls marriage “the Great Sacrament”. This Great Sacrament makes present the love of Christ for his Church. Christ loved us when we were his enemies, when were sinners. He washed us with his blood to make us appear before him as holy and immaculate bride. (Cf. Eph 5:21-33)
Christ chose to be born and grow up in the bosom of the holy family of Joseph and Mary (CCC 1655). The holy family of Nazareth, therefore, is the model for all Christian families. Pope Francis also invites us to imitate Mary and Joseph who welcomed Jesus in their home.
“Each Christian family can first of all — as Mary and Joseph did — welcome Jesus, listen to Him, speak with Him, guard Him, protect Him, grow with Him; and in this way improve the world. Let us make room in our heart and in our day for the Lord. As Mary and Joseph also did, and it was not easy: how many difficulties they had to overcome! They were not a superficial family, they were not an unreal family. The family of Nazareth urges us to rediscover the vocation and mission of the family, of every family. And, what happened in those 30 years in Nazareth, can thus happen to us too: in seeking to make love and not hate normal, making mutual help commonplace, not indifference or enmity.” (Audience, Wednesday, December 17, 2014)
The Second Vatican Council has called the family “The Domestic Church”. “It is in [the family] that the father of the family, the mother, children, and all members of the family exercise the priesthood of the baptised in a privileged way ‘by the reception of the sacraments, prayers and thanksgiving, the witness of a holy life, and self-denial and active charity.’ Thus the home is the
first school of Christian life and ‘a school for human enrichment.’ Here one learns endurance and the joy of work, fraternal love, generous – even repeated – forgiveness, and above all divine worship in prayer and the offering of one’s life. (CCC 1657)