All my life I've yearned for deep friendship. I had companions. I wasn't particularly lonely. But somehow my friendships never satisfied the longing I felt for something more.

I wanted not just what Anne of Green Gables calls a bosom friend, but a mentor, a spiritual director, a guide. In my 20s, I remember feeling I should be satisfied by girls nights out and shopping trips and joking about promiscuous sex—all messages confirmed by the pop culture of the time (I liked Sex and the City as much as anyone). I tried, but I always came home feeling drained and guilty that I hadn't enjoyed it more. I felt like a failure and a traitor to my sex.

The truth is I wanted Christian friends—the kind C.S. Lewis had among the Inklings, who met regularly in the pub to discuss God and miracles and the meaning of everything. But when I looked to church, I saw women walking together in a sweet, pious sisterhood for which I felt completely unfit.

When you search the classic literature of friendship, you find that for Aristotle, Aelred of Rievaulx, Montaigne, Emerson, and much later, C. S. Lewis, true friendship was much more than the companionship often mistaken for it. The friendship we find in their writings will probably not be found in a chat room or at girls' night out or the ladies auxiliary. It is as rare as it is intense—Montaigne thought you might find one true friendship in every 300 years. It doesn't necessarily, as the aphorism tells us,last forever. And yet it has the power to bring us into contact with the eternal, and is therefore inherently spiritual.

By spiritual, the writers of the classics did not mean that friendship in itself is sweet, holy, or pious. What makes a friendship spiritual is not its own perfection, but its origin and direction. A friend, as the Book of Sirach tells us, is a "sturdy shelter" on the path. The friend is also a "treasure beyond price." But friendship is not the ultimate gift or end. A spiritual friendship is always reaching out for what the friendship is really about: God.

These classic writings on friendship—all written by men—often imply, and sometimes boldly declare, that this type of friendship isn't possible for women. Montaigne wrote: "the ordinary capacity of women is inadequate for that communion and fellowship which is the nurse of this sacred bond; nor does their soul seem firm enough to endure the strain of so tight and durable a knot."

If Ruth and Naomi are any indication, women's souls have endured the strains of tight and durable knots since time immemorial. In fact, a landmark UCLA study confirmed what women have always known: we are hardwired for friendship. In addition to the typical "fight or flight" response to stress all people have, research suggests that women alone have an alternative "tend and befriend" response. In difficult times, women seek out other women. Friendship reduces our stress levels and, the authors of the study posit, accounts for longer life spans in women.

And yet friendship by any definition is on the decline. Any Internet search for "friendship in the digital age" or "virtual friendship" will call up countless commentaries worrying that our relatively flip and impersonal digital exchanges are leaving us ever connected and yet terribly lonely. Spiritual friendship has fared no better. Though it has historical roots in Christianity, it now reeks of self-help and bland new age therapy. As Mark Vernon writes in The Meaning of Friendship, "type 'soul friends' (or even worse, 'soul mate') into any Internet search engine, and some of the most syrupy aphorisms on friendship will be returned for your edification."

We haven't just seen a decline in friendships; we've lost the seriousness with which we once treated the idea.

Near the end of grad school, I found another woman who was also obsessed with God and death. She was a writer too. When we both moved to different states after graduating, we began a correspondence—not merely as a way to continue a new relationship that had been cut short, but as a way to keep wondering about God together. The letters became our Oxford pub, where we met almost daily to confess our desires and our doubts, to rant, to question, and when tragedy struck, to curse and grieve.

After five years of writing, we had achieved such an intimacy that it seemed insufficient to call this woman a "friend," or even my "best friend." I'd found the bond I'd been seeking since childhood. But there seemed no right word to describe it.

Unlike Montaigne, I don't think my experience can possibly be all that rare. I'd wager many of us yearn for something beyond aphorisms, beyond companionship, beyond even the biological need we have for relationship with others. But we struggle to understand our desire beyond what entertainment and advertising and even classic literature have told us is desirable and normal. We sense that there is greater possibility, and there is.

Jessica Mesman Griffith is the author, with Amy Andrews, of Love and Salt: A Spiritual Friendship in Letters (Loyola Press).