A: This important question is perhaps one of the most commonly misunderstood distinctions regarding the Church’s teachings on marriage.
A “natural marriage” is a marriage that a man and a woman have when they enter into a marital relationship containing all the natural elements that God has put into what marriage is. Thus, even if they are not baptized Christians, they are truly a husband and a wife.
A “sacramental marriage” is when two baptized persons have a valid natural marriage and, because they are baptized, that marriage is now also a sacrament of the Church.
It is important to remember that marriage is first a “natural institution,” that is, it is established by God as a good in creation, endowing it with its own proper laws (cf. CCC 1603-1605 and Gaudium et spes, 48). Written by God into the nature of man and woman, the common and permanent characteristics of marriage can be seen and understood by all. However, like other aspects of the natural moral law, it is not always transparent everywhere and with the same clarity due to ignorance, cultural biases, sin, etc. Nonetheless, where these natural law elements of marriage are followed, a human person, whether baptized or not, is able to enter into a true, valid marital relationship as God designed it to be. Contrarily, where any one of these natural aspects is missing, the relationship cannot be considered a valid marriage because it would lack an essential element that would have established it as a true act of marrying.
Canon 1055 identifies marriage’s essential, natural law elements: being between one man and one woman, established by and between themselves (through consent), as a partnership for the whole of life, for the good of the spouses and the procreation and education of offspring. Thus, whether a person is Catholic or not, baptized or not, to have a valid marriage, these natural law elements must be present. If they are, the marriage is a valid natural marriage. If they are not, the marriage cannot be considered valid.
Through Original Sin, the world, including the natural institution of marriage, is affected by disorder (cf. CCC 1606-1608). Into this fallen world, Jesus Christ, the long-awaited Messiah whose relationship with his People is seen as a nuptial covenant (cf. CCC 1612), raises the natural institution of marriage to the dignity of a sacrament (cf. CCC 1613).
Thus, the marriage of two baptized persons, in addition to being a valid natural marriage, is now one of the seven sacraments of the Church with all that being a sacrament entails, including being an efficacious sign of Christ’s presence, a channel of the grace of the Paschal Mystery of Christ, etc. (cf. CCC 1641-1642).
Since baptism is the gateway to the sacraments and is necessary for the valid reception of the other sacraments (canon 849), it is necessary that both parties in a marriage be baptized in order for their valid natural marriage to be also a sacrament of the Church. In fact, canon 1055 §2 reminds us that a valid matrimonial contract (i.e., a valid natural marriage) cannot exist between baptized persons without it also being a sacrament, thus becoming a “sacramental marriage.” When this is the case, the essential properties of marriage, namely unity and indissolubility, obtain a special firmness by reason of the sacramental graces involved (cf. canon 1056).
—Answered by Benedict Nguyen, Diocese of Corpus Christi, TX