The main problem with attempting to differentiate between your soul and your spirit is that first you must believe that you have both. If as an atheist you believe that neither actually exists, then the whole discussion ends before it begins.
After all, no scientist has ever discovered either in anyone’s body or brain. In fact, no scientist has ever actually discovered “the mind” in terms of finding something visible or otherwise identifiable. Or, as British philosopher Gilbert Ryle ridiculed René Descartes’ mind-body dualism, no one has ever discovered “the ghost in the machine.”
And yet, we all know that, visible or not, there is something inside of us, within us, that defines who we are, that thing that makes each of us an individual, sets us apart from others, and contains our core selfhood. Many people, especially those of a Jewish or Christian persuasion, call this something their soul or their spirit, using the terms synonymously. Others argue no, that we each have both and the two are distinguishable from one another.
If you look the two words up in a dictionary, you likely will find something akin to the following:
Soul: the spiritual or immaterial part of a human being or animal
Spirit: the nonphysical part of a person which is the seat of emotions and character
They are almost but not quite the same thing. And these are just two examples of the plethora of definitions you likely will find for these two terms.
Both the Jewish and Christian bibles tell the creation story. In it, God created Adam out of earth and dust, but then blew “the breath of life” into him. Some religious scholars interpret this as meaning that when Adam became a man, he became a living person or soul. For such scholars, the word “soul” therefore signifies something earthly. The “breath of life,” on the other hand, signifies something else given to Adam by God after he created him. This subsequent something requires a different word, and that word, they argue, is “spirit.”
If you adhere to these parameters, then your soul, like everything pertaining to the earth, is the intangible part of you that can and often does change over time. It is that part of you that perceives the world through your senses and feelings. It can be influenced by your associations with other people and what they think and/or say about things. In other words, your soul is earthly. It has nothing to do with God.
Your spirit, conversely, is the intangible part of you that connects with God, but only if and to the extent you open yourself to this connection. If you do, God “speaks” to you through your spirit. Since God by definition is eternal, this makes him and his messages to you likewise eternal and unchangeable. In other words, your spirit is, for lack of a better word, heavenly instead of earthly.
Carrying these arguments through to their logical conclusion, if you allow God to give you a glimpse of the eternal, your earthly soul can find purpose and peace here on earth. No longer do you need to shift and change focus with each earthly wind that comes along. Instead, you can rest secure in the knowledge that your existence on earth, i.e., your soul, is only temporary, but your eternal existence, i.e., your spirit, will remain throughout time, free and distinct from other people and the unrest they often bring to your soul.
That “breath of life” presents us with a two-edged sword. Because of our free will, we can choose to limit ourselves to the concerns of our earthly soul, or we can choose to open our spirits up to the unfathomable possibility of eternal life.