Isn't a religion also a part of self-identity? Doesn't it relate to your culture? Doesn't it carry the same importance as the identity of where you hail from? Does religion also relate to nationalism which itself is a kind of religion? If Theodore Herzl was a committed atheist, then what did made him (or why did he become) the founder of Zionist movement, a movement which had its entire infrastructure built on the religion, and demand for a Jewish state? Isn't it a prime example of religion used as an identity? What is a religion? A connection between God and man, or perhaps living life through divine laws? Or, perhaps for most people it is just a part of identity?

"abandonment of a religion" assumes that the person was affiliated with some particular religion to begin with, and he or she has abandoned it.

"abandonment of religion" means that the person has given up on the concept or idea of religion, perhaps after serious examination of an array of possibilities. Not everyone is born into a religion. An atheist family would not bring up their children in any particular religious faith. But when the children are able to make decisions of faith for themselves, they might decide to join a religion, or abandon the entire concept of religion.

But technically atheism comes from 2 Greek roots: "a" - meaning without, and "theos" meaning "god".  So literally, it is defined as the belief that there is no higher power.  It is part of the word family with monotheism (mono: one), and pantheism (pan: many).  Then there is the Latin-derived  word "deism" - from "deos" meaning "god".  The similarity of theos and deos is not accidental!  Deists believe that there is a higher universal power, but they do not believe according to the teachings of a formal religion.