Is a Christian permitted to date or marry a non-Christian?
The Bible's teaching on this question is simple, clear and unequivocal: a believer may only marry another born-again believer. To do otherwise is a sin that carries potentially life-long, negative spiritual consequences and personal hardship. Paul says:
The command of scripture is that a believer not be bound together with unbelievers. The Greek word translated into bound is heterozugeo, which literally means to be yoked together in a single team.
When a farmer prepared to plow his field, he yoked two oxen together to pull the plow blade through the soil. The farmer would chose a pair of animals that were similar in size and strength so that they would pull together in unison, creating straight lines in the field. If the oxen team were imbalanced - if one ox were stronger or larger than the other - then the team couldn't pull with equal power. The oxen team would eventually go astray, leaving the straight rows and meandering off course.
So the concept Paul teaches us is that a believer must not become yoked together with someone who does not agree with the believer's perspective of God, faith, godliness and obedience to Christ. If we become bound with an unbeliever in a solemn agreement (i.e., a marriage covenant), then we are unequally yoked and likely to be pulled astray from an obedient walk with Christ.
Some Christians marry unbelievers thinking they will be the stronger "ox" in the relationship, pulling their unbelieving spouse into the straight and narrow path of obedience to God, but the Bible teachers that obedience is always predicated on faith in the Gospel:
Furthermore, no one can persuade or force an unbeliever to become born-again. Spiritual conversation is a change that God alone produces in the heart of an individual, and therefore marrying an unbeliever in the expectation of converting them is a foolish and presumptuous decision. Paul says:
Paul advises a believer who is married to an unbeliever that they cannot presume they will "save" their spouse. So if a believer marries an unbeliever with the expectation of converting their spouse, they commit a sin, because they openly disobey the word of God without cause to expect success. Moreover, they are signing up potentially for a life of spiritual difficulty, which carries eternal consequences.
Since the unbeliever cannot please God without faith (and the believing spouse has no expectation of changing their situation), then by necessity the unbeliever will become the stronger "ox" in the relationship. Said another way, the believer won't steer the unbeliever into godliness; the unbeliever will steer the believer away from obedience. This is why Paul quotes in 2Cor 6:16-17 from the Law in reminding the church that the Lord ordered believers to separate themselves from committed associations with unbelievers to avoid their negative influences.
In summary, a believer may not marry an unbeliever. If he or she does so, the believer is sinning, and the consequences for that sin will be lasting and potentially devastating. Marrying for love is a romantic notion, but romance and physical attraction are fleeting, while the spiritual consequences of disobedience to God's word are eternal.