Most children think of God as a nice old man with a long white beard, sitting on a chair on a cloud in the sky. As we grow older and more sophisticated in our knowledge of the world, we can no longer believe in such an image of God. If we continue to study religion, then our God-idea can grow and mature along with us. If we abandon religious study after Jewish bar mitzvah or Christian confirmation, then we are likely to retain a childlike notion of God as the rest of our mind matures. Many professed atheists reject nothing more than their own childish idea of God.
Some atheists feel obligated to profess atheism because sophistication demands cynicism.
In this category, we may place many bons vivants as well as many scientists who may fear that acknowledging God will compromise their chances of advancement in their profession. My uncle Herb was a distinguished physicist, university professor, and administrator at the National Academy of Science. On his deathbed, he told me of the depth of his religious faith, despite having been raised to be an atheist by his father, my grandfather. Scientific study convinced him, as it convinced Albert Einstein, that there is a higher power beyond the created universe. Of course God is, by definition, beyond measurement, which means that science can neither prove nor disprove the existence of God.