The Catechism of the Catholic Church

http://www.vatican.va/archive/catechism/ccc_toc.htm

SECTION TWO - THE SEVEN SACRAMENTS OF THE CHURCH
Article 7
The Sacrament of Holy Matrimony

http://www.vatican.va/archive/catechism/p2s2c3a7.htm

1601
"The matrimonial covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament."

1603
"The intimate community of life and love which constitutes the married state has been established by the Creator and endowed by him with its own proper laws. . . . God himself is the author of marriage."
"The well-being of the individual person and of both human and Christian society is closely bound up with the healthy state of conjugal and family life."

1614
The matrimonial union of man and woman is indissoluble: God himself has determined it: "what therefore God has joined together, let no man put asunder."

1626
The Church holds the exchange of consent between the spouses to be the indispensable element that "makes the marriage." If consent is lacking there is no marriage.

1640
Thus the marriage bond has been established by God himself in such a way that a marriage concluded and consummated between baptized persons can never be dissolved. This bond, which results from the free human act of the spouses and their consummation of the marriage, is a reality, henceforth irrevocable, and gives rise to a covenant guaranteed by God's fidelity. The Church does not have the power to contravene this disposition of divine wisdom.

1641
This grace proper to the sacrament of Matrimony is intended to perfect the couple's love and to strengthen their indissoluble unity. By this grace they "help one another to attain holiness in their married life . . .

1644
The love of the spouses requires, of its very nature, the unity and indissolubility of the spouses' community of persons, which embraces their entire life: "so they are no longer two, but one flesh."

1647
Through the sacrament, the indissolubility of marriage receives a new and deeper meaning.

1649
Yet there are some situations in which living together becomes practically impossible for a variety of reasons. In such cases the Church permits the physical separation of the couple and their living apart. The spouses do not cease to be husband and wife before God and so are not free to contract a new union. In this difficult situation, the best solution would be, if possible, reconciliation. The Christian community is called to help these persons live out their situation in a Christian manner and in fidelity to their marriage bond which remains indissoluble.

1650
Today there are numerous Catholics in many countries who have recourse to civil divorce and contract new civil unions. In fidelity to the words of Jesus Christ-"Whoever divorces his wife and marries another, commits adultery against her; and if she divorces her husband and marries another, she commits adultery"-the Church maintains that a new union cannot be recognized as valid, if the first marriage was. If the divorced are remarried civilly, they find themselves in a situation that objectively contravenes God's law.

1660
The marriage covenant, by which a man and a woman form with each other an intimate communion of life and love, has been founded and endowed with its own special laws by the Creator. By its very nature it is ordered to the good of the couple, as well as to the generation and education of children. Christ the Lord raised marriage between the baptized to the dignity of a sacrament (cf. CIC, can. 1055 § 1; cf. GS 48 § 1).

1664
Unity, indissolubility, and openness to fertility are essential to marriage. Polygamy is incompatible with the unity of marriage; divorce separates what God has joined together; the refusal of fertility turns married life away from its "supreme gift," the child (GS 50 §1).

PART THREE - LIFE IN CHRIST

SECTION TWO - THE TEN COMMANDMENTS

ARTICLE 6 - THE SIXTH COMMANDMENT

http://www.vatican.va/archive/catechism/p3s2c2a6.htm

2382
The Lord Jesus insisted on the original intention of the Creator who willed that marriage be indissoluble.¹⁷³ He abrogates the accommodations that had slipped into the old Law.¹⁷⁴  Between the baptized, "a ratified and consummated marriage cannot be dissolved by any human power or for any reason other than death."¹⁷⁵

2383
The separation of spouses while maintaining the marriage bond can be legitimate in certain cases provided for by canon law.¹⁷⁶

If civil divorce remains the only possible way of ensuring certain legal rights, the care of the children, or the protection of inheritance, it can be tolerated and does not constitute a moral offense.

2384
Divorce is a grave offense against the natural law. It claims to break the contract, to which the spouses freely consented, to live with each other till death. Divorce does injury to the covenant of salvation, of which sacramental marriage is the sign. Contracting a new union, even if it is recognized by civil law, adds to the gravity of the rupture: the remarried spouse is then in a situation of public and permanent adultery:

If a husband, separated from his wife, approaches another woman, he is an adulterer because he makes that woman commit adultery, and the woman who lives with him is an adulteress, because she has drawn another's husband to herself.¹⁷⁷

2385
Divorce is immoral also because it introduces disorder into the family and into society. This disorder brings grave harm to the deserted spouse, to children traumatized by the separation of their parents and often torn between them, and because of its contagious effect which makes it truly a plague on society.

2386
It can happen that one of the spouses is the innocent victim of a divorce decreed by civil law; this spouse therefore has not contravened the moral law. There is a considerable difference between a spouse who has sincerely tried to be faithful to the sacrament of marriage and is unjustly abandoned, and one who through his own grave fault destroys a canonically valid marriage.¹⁷⁸