Friendship is a rare and demanding kind of relationship between two beings. In Ancient philosophy, it meant a relationship that was formed and forged between two beings of good and of virtue, and who aspire to wisdom.
Even rarer is spiritual friendship, that privileged relationship between two people, a relationship of one's choosing, but that both aspires to the divine and leads to the divine. Far from simply being an affectionate relationship, it is one involving knowledge, intelligence and growth. Each friend is for the other a factor of transformation and elevation.
Following on from this, Christian spiritual friendship, developed by Christianity in the first monastic communities, is an expression of the love of God. This spiritual friendship between two beings: two men or a man and a woman, tends towards divine love. It is an elevated and uplifting goal.