The main problem with those who profess to be believers dating unbelievers is their misunderstanding of the gospel itself. When we misidentify the nature of our entrance into a relationship with God, we misidentify whether we are in that relationship. When we misidentify ourselves, we misidentify others as well. If I do not know what a Christian actually is, I will likely not know how to identify one as a Christian beyond his or her claim to be one. Hence, if it is the gospel that makes one a Christian, then getting the gospel wrong is simply disastrous to our lives and our ability to glorify God with them. As Luther once said to the illogical moralist Erasmus, "How do I know what to do if I do not know what to believe?"
If you ask the professed believer what the gospel is, he or she will likely tell you that Jesus died on the cross to forgive them of their sins, and if they believe, they'll be saved. At least, you would hope to get this much; but this isn't the entire gospel--at least, not in the way the modern person understands the term "believe." To a modern person, belief is theoretical. As we discussed before, however, belief has to do with allegiance to God. It is entering a relationship with God that norms all other relationships. In other words, our allegiance to God is defection toward the lives we were building at the time we made that allegiance. What I'm speaking of here is repentance, a rejection of the self, a changing of the mind about life, a turning of one's life in a different direction.
Notice that when John the Baptist comes preaching, he says, "Repent, for the kingdom of God is at hand" (Matt 3:2). This is the call of the prophets. But what is interesting is that when the Lord Jesus begins to preach the gospel, His message is the same, "Repent, for the kingdom of heaven is at hand" (4:17). He condemns the cities in Matthew 11 because they did not repent. In Acts 2:38, Peter preaches that the Jews to whom he is speaking must "repent, and be baptized in the name of Jesus [i.e., dedicate one's life to Christ]" in order to have their sins forgiven. He again says, "Repent and return, in order that your sins may be wiped away" (3:19). In Second Corinthians, Paul tells us that there is "a sorrow that is brought about by God that produces a repentance that leads to a salvation without regret" (7:10). And the writer of Hebrews calls the "repentance from dead works "basic teaching about Christ" that makes up the "foundation" of Christianity.
Does this mean that belief is not what the gospel requires? Of course it does. We are told that repentance is a part of belief. Many times the term "believe" will either stand with it or alone. Many times, as we see from some of the examples above, repentance stands alone. This is because repentance and belief are intertwined in Scripture. He who believes, repents. He who repents, believes. One cannot make the biblical step of receiving the gospel without the other. This is because the gospel is commanding our allegiance to Christ. It is calling us to turn away from the self, i.e., the desires and pursuits of the corrupted life we have built (and others have helped to build in us), and turn toward the life that Christ would build for us. This means that repentance is giving up self-direction and giving the direction of our lives and decisions over to Christ.
Hence, where the modern person describes a Christian as someone who believes the facts about the gospel, the Bible describes a Christian as one who has given his or her life over to Christ, who is not merely his or her Savior in a theoretical sense, but is his or her Lord as well. Incidentally, what He is saving, even as Savior, is not merely the person's disembodied soul in the day of judgment, but the person from a life of self-direction and treachery of sin today. Hence, Christ is not my buddy that gives me good advice as I decide what to do with my own little world. He is the Lord of my world. He decides what I will do with it. If He commands, I seek to obey. That is the nature of the true Christian's relationship with Him. That is the commitment one has made. If such a commitment has not been made, then one is merely a catechumen, one who is merely learning about Christ from the outside, and not a Christian, one who is learning from Him within a relationship with Him as Lord. The message of repentance is a message to us of Christ's Lordship that would have our allegiances turned over to Him. Hence, the gospel is "Repent/Believe," not simply "Believe" within the framework of our modern theoretical thinking toward an object.
So what exactly is the promise I made when I became a Christian? I promised to follow Him. I promised to listen and obey Him. After all, it is the person who hears and does what he is told that builds his house on the rock, not merely the person who hears. Those who merely learn without doing are cast away from Him as those who practice anomia "lawlessness," which describes a self rule without an external authority (Matt 7:21-27). The promise that was made was to give up the pursuit of all that is not glorifying and pleasing to Christ.
So can the person who enters into a relationship with an unbeliever in disregard of Christ's Lordship in his life really be called a believer? That depends whether he is aware of what God commands. It is possible to be deceived as a believer. It is possible to sin as a believer. But is it possible to completely reject the relationship of Christ as Lord in life altering ways, and without repentance, remain a believer? I don't think so. In fact, the Bible seems to indicate that to enter into a relationship with an unbeliever is not only a sin, but the sin. It is the sin of rejecting Christ. It is the sin of repudiating the gospel. It is the sin of aligning oneself again with the demonic, self-willed life one was leading before. It is the sin of throwing away one's relationship with Christ, breaking the covenant made with Him through the gospel, and burning the bridge he must cross to be saved. The promises of God in salvation, after all, are promises made to those who make their allegiance with Him through the gospel, not to those who are outside that relationship. To set aside the relationship, then, is to set aside the possibility of one's own salvation and citizenship in the kingdom of God, where the person has opted for corruption over holiness.
Hence, we see why God commands all of those who entered into such relationships with unbelievers to be destroyed, cut off from among His people, rebuked as the enemies of God, and condemned. The entire teaching of the Bible on the matter comes to a head in Paul's statement in First Timothy:
But refuse [to put] younger widows [on the list of financial support], for when they are drawn away by their own affections from Christ, they want to get married, [thus] incurring damnation, having repudiated/rejected their previous faith commitment. (5:11-12)
This refers to young widows who want to get married "in disregard of Christ," that is, in disregard to the commitment they made to be devoted to Him. This does not refer to some commitment to stay single, as Paul tells them that he wants them to "get married, bear children, manage the household" in verse 14. The only problem Paul would see, then, with them marrying is when it is contrary to the will of Christ, and the only marriage contrary to the will of Christ that Paul mentions before is someone who does not marry in Christ (i.e., a believer). In First Corinthians 7, Paul states that a widow is free to marry, "but only in the Lord" (v. 39). To marry an unbeliever is to reject the faith. That is the entire teaching of the New Testament that has been gained from the teaching of the Old Testament. God's will is clear. Christ's will is clear. There is nothing more to say about it. To enter into a romantic relationship, or to even entertain and play with the idea, is to reject, or entertain and play with the idea of rejecting, Christ and one's commitment made in his or her supposed reception of the gospel.
And that is the most important point to be made here. This is a gospel issue, not just some minor life choice that can be made along with one's commitment to Christ. It is a matter of one receiving or rejecting Christ, as Christ will only be Savior when He is Lord (He only saves that which belongs to Him, as that which belongs to Him is a part of Him, and since He is saved, what is a part of Him is saved with Him). It does not matter if it all "works out." The gravest of sins has been committed. Is there no repentance for what one has done? Is there no repentance for what one is doing? Then there is no faith in Christ. There is no gospel of salvation. There is no Christ in one's Christianity. If there was, repentance would be there too. It is the constant companion to the Christian for the rest of his days.
Hence, the "believer" who dates a non-Christian, which is defined by a lack of the same repentance above (not by a mere lack of a profession of belief in facts about Christ), shows him or herself to not be a believer at all, having thrown off the eternal love of Christ to gain what is perishing. He or she joins his or her selves to what perishes because he or she is perishing with it. Like Judas, who came to believe that thirty pieces of silver was more valuable than his commitment to Christ, the unbeliever is put in the place of Christ as well. The unbeliever is the so-called believer's thirty pieces of silver, and whether his profession to have given up all to follow Christ is true will be displayed, not only in whether he receives that bribe, but also in how much he contemplates taking it. The one who professes to be a Christian, and yet pursues a romantic relationship with an unbeliever, in thought or deed, has taken the devil's bait, and as with most traitors, will often see his folly all too late. He has become a kissing Judas. He both proclaims his affections for the Lord and betrays Him all at the same time.
The one who truly loves Christ, and has truly come to know Him, will hear these words and weep in repentance like Peter. The one who does not, and never came to know Him, will weep one day, but in regret looking back, not in repentance looking forward. If you claim to be a Christian, and have entered, or are contemplating entering, such a relationship with an unbeliever, let me simply ask you this, "Which one are you? A Peter, or a kissing Judas? Only what you do now will bare it out.