In his second epistle to the Corinthians, St Paul states:
“Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness? What accord has Christ with Belial? Or what portion does a believer share with an unbeliever?”
Even though Paul may be referring to social connections in general, marriage, as one of the most important “social connections” in human life, is certainly included here.
Marriage binds a man and a woman in a single unit that has physical and spiritual dimensions unique to this status, so special that it is compared to the union of Christ and the Church (Gn 2:24; Mk 10:6-8; Eph 5:25-33). For a marriage to be successful there must be coherence in moral beliefs and a common view on life’s purpose. Without this cohesion the harmonious unification of a couple “as one” in the full sense of the word, would be impossible.
Furthermore, St Paul also says: “Bad company ruins good morals” (1Cor 15:33). A union between a Christian and a non-Christian is wrong not only because it goes against an express command from Scripture (2Cor 6:14), but also because no spiritual bonds will grow from such a distinct spiritual realities, a Child of God and a heathen. But it does not stop there, all differences in ethics, habits, including religious convictions, would become even more pungent when children come into play. Questions like drinking, smoking, nightclubbing, dating, premarital sex, homosexuality, church assistance, and the very concept of God, would be a motive of friction when educating their children, and would end up corrupting the souls of the believing partner and their offspring if Christianity does not take first hand in the relationship.
WHO IS AN UNBELIEVER
A proper unbeliever is a non-Christian person, either from a non-Christian religion, a professing atheist or a member of a sect, or non-Trinitarian organization.
When St Paul spoke of “unequal yoke” or ‘light and darkness”, he was referring to unions between Christians and non-Christians, and by extension, also to the sects. To everyone who does not possess the Holy Spirit.
In modern times, the word ‘unbeliever’ is applied by many, even to other Christians who, despite being Trinitarians and accepting the truths of the Nicaean Creed, are rejected as “heretics” for not belonging to the same ecclesiastical organization we belong to. However, even though to marry within the same religious organization maybe the wisest decision for complete religious coherence; this should not be a strict impediment, since all Nicene Trinitarian Christians are true Children of God, sealed with the Holy Spirit, professing all the truths necessary for salvation. In cases like this, eventually one of the parents will prevail and the children will be educated within the prevalent view, or the children would make the decision themselves at age of consent.
THE ONLY CASE A CHRISTIAN IS ALLOWED TO BE MARRIED TO AN NON-CHRISTIAN
The only time a Christian is allowed to cohabitate with a non-Christian partner, is when one of them has converted, being already married, from a non-Christian background. In this case, the union becomes legitimate and there is no demand for the Christian man or woman to abandon his or her pagan partner (1Cor 7:12-15). However, if the pagan partner abandons the Christian partner, he or she is allowed to file civil divorce and re-marry.