And God in His Word (2Cor 6:14) has told His children to refrain from being 'yoked' with those who have yet to commit themselves to Him (a yoke suggests both the idea of a strong/intense binding and that of permanency, characteristics more than adequate to describe romantic relationships).  Remember that God's goal for His kids are that they be like Him in character, demonstrating all the compassion, mercy, gentleness and overall 'color' in their life which can come only when we focus on His heart and His words day and night.

Efforts to realise this goal of 'Jesus-likeness' will not be aided if our spouse - the dearest person with whom our very lives are shared - is not him/herself going in the same direction or desiring the same end.  It wouldn't be disparaging to say that a non-believer would simply not identify at all with the allegiance a child of Christ ought to give his/her heavenly Father and the actions/behaviours this will require (Lk 14:26).  Furthermore, a 'high' view of love relationships should naturally entail that there be no fundamental differences in the worldviews of the two people in question.

The 'one-flesh' principle represents a joining together not only of mind and body but spirit too, and thereby ought to require a basic agreement about the 'spiritual'.  It would be self-evidentially problematic for the relationship if the husband believes an eternal relationship with a personal God awaits him, whilst his naturalism-inclined wife expects to have her consciousness disintegrated/annihilated at death.

Failing a conversion of the spouse, all this may very probably result in the following 'un-desirables':

• Falling intimacy between the couple due to arguments over ultimate values, what's priority for the family (with regards to money, time and other resorces).

• Falling faith on the part of the Christian (due to compromises or syncretization with the spouse's differing world-view).

(Note: I am not saying that Christian couples never experience diminishing love for each other and God; this is an issue which is increasingly inflicting the church and which ought to be continually addressed and 'diffused'.  However, if this happens in Christian relationships - where two people share a common Lord of Truth and Love - this provides even more reason to be cautious about uniting two people which differ on the matter of what life really means and from Whom all authority and values derive).
 

Objection:  But what about those cases (and there are quite a number) where, AFTER the relationship began, either a) the spouse was converted and/or b) the Christian's faith was strengthened?  Can't we convert a person during the relationship and pray that our faith be upheld?

Although I grant these future possibilities, they however cannot serve as factors in our present decision-making.  If God teaches us one thing, we ought not to act against it on the basis of our 'optimism' for the future.

I would also think that these 'success cases' show not the wisdom of human decisions but the grace and power of God at work in human lives.  God works in spite of our tendency to go against His will; God has many Plan Bs' which He activates (with love yet sadness) as non-preferred contingencies because people reject His initial - and usually very explicit! - Plan As'.

However, the fact that God has contingency plans is in NO WAY a trigger for us to challenge/question His primary directives.  Playing with fire is never advisable even though there's water and a first-aid kit around.  I'm also very sure that the cases in question have ALL involved lots of unnecessary hurt, pain, and emotional distress for both parties.  That things turned out well eventually is an occasion for more - and not less! - attention and obedience to what God has instructed us on beforehand.
 

Practical Steps and Conclusion:

True romantic love between guy and girl is a heavenly premium, the joys and wonders of which our Father wants everybody to experience.  Nevertheless, the full consumation of human sexuality needs to be founded upon and 'underwritten' by the mutual love of God which the couple has.  I fully believe that me and my spouse can only love and fulfil each other to the extent that our hearts have been surrendered to God and His grace and strengthening 'fed back' into our relationship in a continuous cycle of love.

God truly (unlike certain corporate parties, *smile*) wants to give everyone the BEST there is in Life and so He leaves us with instructions which - although seemingly 'unfair' or 'un-cool' - we ought to give Him the 'benefit of the doubt' on (remember that no one else knows Loving like He does! *grin*) and thus take to heart.  With that said, allow me to recommend the following 'practical' steps:

1.  Share Jesus with your friend, tell what God has done in your life and what He can do for his/hers (needless to say, your words ought to be continually 'acted out' in real-life, *smile*)

2.  Explain gently why you'd rather be in a relationship with someone who shares your faith and passion for God, and make it very clear that this is a non-negotiable 'standard' below which your heart will not commit.  (I can only expect the person, assuming he/she genuinely cares about and respects you, to be moved by such a desire/principle, regardless of the accompanying disappointment).

3.  PRAY for the person, and believe in your heart that it would be best for BOTH of you that he/her comes to know and love the Lord AS A PREREQUISITE for sharing your lives - and love - together.
 

Hope this helps,
AL