1 Corinthians 7:39 speaks of our obligation to be married “in the Lord.” The Catholic Church understands this to mean a religious service according to the laws of God.
Jesus gave the apostles and their successors his authority to teach us how to follow the laws of God in all areas of our lives (including marriage):
“Jesus came and said to them, ‘All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age.’” Matthew 28:18-20
Thus, the Catholic Church, in obedience to God, has laws concerning marriage and obliges her members to follow them.
The importance of refraining from “unlawful marriage” is brought out in Acts 15:28-29, which records the directive of the Apostles at the council of Jerusalem, in 50 A.D. This decision was presented as having the divine authority of the Holy Spirit and was binding on all of the faithful:
“It is the decision of the holy Spirit and of us not to place on you any burden beyond these necessities, namely, to abstain from meat sacrificed to idols, from blood, from meats of strangled animals, and from unlawful marriage. If you keep free of these, you will be doing what is right.”
In other words, the Holy Spirit speaks through the official voice of the Church in matters regarding what is considered a lawful marriage. Matthew 18-15-18 shows that Christ intended his Church to exercise authority on his behalf in deciding judicial matters of consequence (like marriage) for his disciples:
“If your brother sins against you, go and tell him his fault, between you and him alone. If he listens to you, you have gained your brother. But if he does not listen, take one or two others along with you, that every word may be confirmed by the evidence of two or three witnesses. If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector. Truly, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.”
Scripture again presents the importance of having a lawful marriage in the story of John the Baptist, who was imprisoned (and later beheaded) because he told Herod it was not lawful for him to marry his brother’s wife. (See Matthew 14:3; Mark 6:17-18) This passage reveals that it is the Church (symbolized by John the Baptist), not the state (symbolized by Herod), which has the authority to determine what constitutes a lawful marriage in the eyes of God.
Marriage is a sacrament and the Church is the custodian and steward of the sacraments (see 1 Corinthians 4:1; Ephesians 3:2; Titus 1:7) and the one who determines their validity.
Around the year 110 AD, or earlier, St. Ignatius of Antioch, wrote a letter to St. Polycarp, which included the following advice concerning marriage:
“Those who marry should be united with the bishop’s approval, so that the marriage may follow God’s will and not merely the prompting of the flesh. Let everything be done for God’s honor.”
St. Ignatius lived during the time of the Apostles. He was a disciple of Sts. Peter and Paul and of St. John the Evangelist. St. Chrysostom tells us that St. Peter appointed him as the third bishop of Antioch where he was bishop for forty years before his martyrdom. (St. Peter himself was the first Bishop of Antioch.) Both he and St. Polycarp were martyrs.
Marriage is both a public and sacred institution. It is not just between the husband and wife. It serves as the foundation of the family, which protects the rearing of children and serves as the basic building block of society. The well-being of society is directly related to the well-being of the family, which is founded on marriage.
The Church obliges its members to contact marriage according to its laws for the following reasons (CCC 1631):
- Sacramental marriage is a liturgical act. It is therefore appropriate that it should be celebrated in the public liturgy of the Church;
- Marriage introduces one into an ecclesial order (a state of life in the Church), and creates rights and duties in the Church between the spouses and towards their children;
- Since marriage is a state of life in the Church, certainty about it is necessary (hence the obligation to have witnesses);
- The public character of the consent protects the “I do” once given and helps the spouses remain faithful.
- “So that the ‘I do’ of the spouses may be a free and responsible act and so that the marriage covenant may have solid and lasting human and Christian foundations, preparation for marriage is of prime importance.” (CCC 1632)
The following excerpt is taken from an Apostolic letter written by Pope Saint John Paul II: “On the Role of the Christian Family in the Modern World” (Familiaris Consortio). The last line of this quote speaks to the fact that a priest, even while expressing great compassion for someone in an invalid marriage, is none-the-less not free to admit them to the sacraments.
Pastoral Action in Certain Irregular Situations (Familiaris Consortio)
79. In its solicitude to protect the family in all its dimensions, not only the religious one, the Synod of Bishops did not fail to take into careful consideration certain situations which are irregular in a religious sense and often in the civil sense too. Such situations, as a result of today’s rapid cultural changes, are unfortunately becoming widespread also among Catholics with no little damage to the very institution of the family and to society, of which the family constitutes the basic cell…
Catholics in Civil Marriages
82. There are increasing cases of Catholics who for ideological or practical reasons, prefer to contract a merely civil marriage, and who reject or at least defer religious marriage. Their situation cannot of course be likened to that of people simply living together without any bond at all, because in the present case there is at least a certain commitment to a properly-defined and probably stable state of life, even though the possibility of a future divorce is often present in the minds of those entering a civil marriage. By seeking public recognition of their bond on the part of the State, such couples show that they are ready to accept not only its advantages but also its obligations. Nevertheless, not even this situation is acceptable to the Church.
The aim of pastoral action will be to make these people understand the need for consistency between their choice of life and the faith that they profess, and to try to do everything possible to induce them to regularize their situation in the light of Christian principle. While treating them with great charity and bringing them into the life of the respective communities, the pastors of the Church will regrettably not be able to admit them to the sacraments.
(ref. CCC 2030-2037)
Catholics are bound by the laws of the Church concerning marriage. This authority comes from Christ and is revealed in scripture. 
Marriage between two baptized parties is a sacrament of the Church. As such, the Church has the authority to determine how her Sacraments will be celebrated.
The Church states that to have a marriage recognized by the Church, the man and woman must be married before a priest, deacon (or in extreme cases, a layman with the requisite approval) and at least two witnesses. This requirement may be dispensed, but certain procedures must be followed to do so.
If a Catholic is married civilly, or in a non-Catholic Church without the necessary dispensation, then this “marriage” is invalid.
Thus, he would be in the same position as an unmarried person who is engaging in sexual relations.
Just as an unmarried person who engages in sexual relations cannot be admitted to Holy Communion because of the requirement that one must be in the state of grace to receive Holy Communion and, furthermore, cannot receive Absolution if he refuses to abandon the sin of fornication, a Catholic who is married in a non-Catholic Church without the necessary dispensation cannot approach Confession and Holy Communion because of the continuing state of grave sin.
Catholics who find themselves in this state should speak with a priest about how they can reconcile their situation so that they may again receive the sacraments.
 The scriptural basis for the Catholic Church’s authority to declare that all Catholics are “bound” by her laws, is shown by the words of Jesus himself in Matthew 18:15-18.