I often say to people that sustainability is a spiritual practice. What do I mean by this? I mean the simple truth that both of them, when reduced to their most fundamental, core meaning, are about how we are all interconnected and how everything we do affects everyone else.
For many, the word "spirituality" conjures up images of religion or meditation, and it's true that those are aspects of spirituality. Other people often juxtapose spirituality and religion, denying that they are "religious" and adopting instead the word "spiritual." I don't think most people really know what they mean when they say that, but when I hear that, I most often assume they are trying to suggest that they don't follow a rigid dogma, but somehow they still are in tune with certain aspects of being human that we might label "spiritual." What they want to convey is that they are not closed to a sense that they are connected to something larger than themselves, and that they have enough of an introspective side to cultivate that connection.
Spirituality, in this sense, is the understanding that none of us exists in isolation. In fact, none of us exists without another. This is true in a literal, physical sense: we all required a mother to give birth to us, so there's at least one other person in this world upon whom our life quite literally depended. Througout our lives, nothing in our lives was ever done entirely in isolation; everything is a product of multiple lives intersecting. It is also true at the level of language: the word "I" only makes sense in relation to the word "you." There's no sense of a person, separate and unique from others, without those others. To say "I" presupposes a "you" or a "they" -- and it also means that there is always a "we."
Sustainability is really the same thing: it is the recognition that, as physical beings, we depend upon the living energy of this planet and the efforts of all others to sustain our physical life. Each time we eat, drink, take the subway, drive a car, go to work, we are entering a matrix of relationships where everything that comes into our physical reality was the product of a connection with another person. Sustainability asks us to recognize that physical reality and honor it by caring for the planet's resources, treating workers fairly and paying wages, cleaning up our waste, etc. -- in other words, to honor the connections we have with all others as expressed through our physical reality.
Spirituality takes the view that we are all connected by virtue of the fact that, as living beings, we all have hopes, dreams, fears, and failures. Everybody is trying to live the best life they can, fulfilling their wishes, wanting to be loved and taken care of, to feel that they belong. Spirituality asks you to examine how you treat others -- do you judge or accept? do you forgive or hold grudges? do you love or hate? If you regard someone as having an equal right to the fullest life possible, then it is impossible not to also want them to have all of the resources necessary to live out that life. That includes future generations, not just everyone living at the present moment.
Sustainability, then, is a spiritual practice -- it asks you to honor your connection to your fellow humanity. And spirituality cannot but lead to sustainability, for you cannot wish your fellow humanity access to a good life and then take actions that deprive them of the material means to achieve it.