I'd like to hypothesize one more thing. I consider "spirit" to be energy, and I personally feel that God might be the source energy of all, and this source pervades all other energies and also extends beyond it, and all energies are born from this source energy. Now, spirit is an energy that is born from THIS source energy [God] and both initially shared the same frequency. So, an individual was inherently an energy born from the source energy [God] and shared the same frequency.
But, when this spirit took form of a "human", an individual and his spirit was imprisoned, cemented inside of matter, material body called "human body". This materialistic prison of human beings gave birth to human nature. Human nature is a part of a specific individual's overall nature [with spiritual nature being the real, true nature dormant in human form]. Human nature is egoistic, selfish, uncaring, brutal, dark, just like the mother nature from which it is created. And due to this egoistic nature and dark side of human, our minds and spirits are corrupted by egos interference from it's initial pure form, and the initial frequency of our spirits has been tweaked, becoming misaligned with the frequency of our true energy selves [our spirits]. And this mismatch has led to discordance and separation from the source energy, which is God.
Spiritual growth is just exercises we do to achieve the initial resonation between the source energy [God] and our TRUE energy selves [spirit]. When this resonation is achieve, peace is achieved. This resonation will lead to reunification with the source energy, God, and that'll be heaven/moksha/whatever. Religions/spirituality/etc. just provides us with exercises and structure to put this spiritual exercises in practice to achieve this resonation with God.
Just my two cents.