With paragraph 17 we begin with the real meat of this letter - a full understanding of what the family is and a complete vision for its life.
God's plan for the family shows us what the familys is and what it is called to do. The mission of the family flows out of its identity. Every family holds a responsibility which comes directly from its own dignity. So, the Holy Father calls families to become what they are.
Every family is called to embrace its inner truth, for in doing this the family will embrace its mission, a mission which finds its fulfillment in the Kingdom of God. The inner truth of the family is that it is called to guard, reveal and communicate love - it is called to be a living reflection of and a sharing in God's love for humanity, and the love of Christ for the Church.
Looking at the concrete life of the family we can see how it is called to embrace its mission. Love has a fourfold mission in the family:
1. To form a community of persons.
2. To serve life.
3. To participate in the development of society.
4. To share in the life and mission of the Church.
Forming a Community of Persons.
This is the first task of the family. The power for this task is love, for it is love which gives meaning to human life. The love given and received between members of a family is the essential basis of this community.
The foundation of family communion is the communion between husband and wife. Marriage taps the natural basis of human love, enabling spouses to fully express the true inclination to love which the human person has. In Christ, God takes this basic human inclination and confirms, purifies and elevates it in the Sacrament of Marriage. In this Sacrament, the Holy Spirit enriches human love with Divine Love. This is how marriage becomes a Sacrament of Christ's love for the Church. The gift of the Holy Spirit transforms the love of the spouses, bringing their lives together on all levels - body, character, heart, intelligence, will and soul, and reveals a new communion of love between them. This is what Christ does with marriage between baptised people.
Marriage then, reveals the personal dignity of husband and wife - a dignity acknowledged by mutual and total love. Polygamy, on the other hand, is a direct negation of God's plan.
(Paragraphs 17 - 20)