The Bible clearly teaches that a Christian must marry a Christian if he or she is to be married. Many use 2 Corinthians 6:14 as proof that it is a sin for a Christian to marry a non-Christian. It says, "Do not be yoked together with unbelievers. For what do righteousness and widkedness have in common? Or what fellowship can light have with darkness?" In the context, this passage is not a teaching about marriage. It is a general principle that Christians ought to be cautious about involving themselves too deeply in situations which tie them too strongly with non-Christians. We can apply this to business associations, contracts in general or even politics. In general, the stronger the connection, the more cautious a Christian ought to be. In a business contract, Christians ought to be cautious abotu ones which might force them to be involved in sinful behaviour. Now, although this passage is not necessarily discussing marriage, surely the marriage "contract" is the most obvious application of this principle! In what human interaction are more closely yoked together than marriage? Although 2 Corithians 6:14 is not a commandment not to marry a non-Christian it is very strong advice not to do so.
Actually, it is 1 Corinthians 7:39-40 is a commandment from Paul that Christian women who marry must marry a man who "belongs to the Lord." Most conservative scholars have interpreted this as a commandment that Christians ought to marry Christians only. Unlike the very strong advice, based on principle in 2 Corinthians 6:14, here we have a commandment. In any case, it is clearly a Christian teaching that marriage of a Christian ought to be to another Christian. What about those who disobey this command? Do they go to hell for this sin? Ought they to be disfellowshipped for such a sin? My personal advice is that the church ought to very strongly teach Christian marriage to Christians only. Also, the church should only be willing to perform marriage of Christians to Christians, but I do not believe those who disobey this commandment lose their salvation by this sin, although they do endanger their spiritual health by this mistake.
As to whether this advice/command applies to marriage within one particular fellowship, the answer is clearly no. A member of a particular fellowship is not forbidden from marriage to another saved person from a different fellowship. Anyone who teaches it is a sin to marry another true Christian from another group is clearly not speaking biblically. However, I can see that it might be good advice for a member of a fellowship to marry within that fellowship for reasons of preserving Christian relationships. In other words, if a brother of sister leaves the fellowship God put them into in order to marry a partner from another fellowship, this will clearly involve breaking some key Christian relationships, which can be risky, especially for a young or weak Christian. Therefore, my advice is that it is normally best to marry in a way which does not cause us to completely reestablish Christian relationships–in other words, I advise marriage within our own group as the wisest idea in most situations, but this is certainly not a command.