A spiritual atheist is spiritual only in the partial sense. Because spirituality needs a basis, and in atheism or the philosophy of atheism there is no basis for spirituality. Spiritual atheism is like a rootless tree. Religion gives us a basis for spirituality. For example, life is full of disadvantages. When an atheist faces these disadvantages, he may fall prey to frustration. However, a believer will not become frustrated, because his belief in God provides him unlimited conviction and trust, which an atheist does not possess.

Source: The Reality of Monotheism (Haqeeqat e Tawheed)