If a companion with whom one is sharing one’s life is also walking the same spiritual path then there is completeness in the relationship. However, it must be noted that people walking the spiritual path do not always make good companions for life. It is a sort of paradox but the reason is that the ego shell has to ripen to feel the stifling pressure which eventually finds release in breaking free from the limitations of the mental life. Therefore, there is a phase when the nascent spirituality is yet to blossom and the struggle with the ego-self is going on, a struggle that often takes long unless there is a special Grace. This inner struggle tends to spill over into one’s outer life and relationships.
To put it in modern psychological terms, people who turn towards spiritual life (not the religious but the true spiritual) are highly individualized beings. This makes them often less malleable to others and more focused on realizing their own goals. Of course, the exceptions are the bhaktas and those who have already travelled deep enough or have found their souls. But true bhaktas are rare and the soul is not easy to find. Hence relationships between two persons who are spiritually oriented is far from easy.
Besides this yoga that aims at a total overhauling and transformation of life brings its own set of challenges and upheavals of nature which are not easy to handle. One has to stay focused on the real goal rather than scatter and disperse in outer things that a relationship normally demands. That is one of the reasons why in most places of spiritual practice, including Sri Aurobindo Ashram the sadhakas are advised to live alone even though they have to be with the whole world outwardly.
Whatever it be these things cannot be decided by a mental logic nor can one find a relationship that is ideal in all aspects. Therefore, one has to keep the grain of gold while leaving the chaff aside. The most important thing in a relationship is love. If that is there one should keep the relationship since love itself is very rare in this world. In fact, true love is indeed a divinity in its own right. If true love is there or at least an effort towards it then one is already striving on the Path. It does not matter whether one outwardly believes in god or not or is into some formal practice. It is enough if one loves and is ready to strive towards making this love more and more beautiful and harmonious and true.
In fact any aspect of human effort can be turned into a yoga if one strives to discover the deeper and greater and higher possibility within it. So much more important than theism or atheism is the urge for progress towards a greater and greater perfection (not ambition that only confounds and diverts this urge into endless meanderings), a greater and greater possibility, a deeper and deeper inquiry into the truth of things and not just the appearances in which our life is caught up incessantly. So if your companion is an atheist she can always be led through this door of progress and perfection.
Even if you have a companion who does not so strive and is satisfied with life, you have somebody who loves you. If a flower of true love blossoms in your life, that is enough to build beautiful things within the range and scope of our humanity. And with a sole aspiration supporting the journey of togetherness, who knows an opening may eventually come in your partner. Perhaps this is the part you have to play in your partner’s life and hence it must be played well as Her channel and instrument. Meanwhile one can always continue with one’s aspiration undeterred by outer circumstances taking them all as a means to develop equanimity and challenges on the Path.