Having come to some understanding, or definition, of what a religion is I can understand how a person’s religion really doesn’t define their beliefs. PauaPrincess pointed this out in Is your religion your belief? This might have some consequences as we normally do assume religion defines belief.
Seeing religion as a tradition, rather than a belief, helps explain why religions become identified with different countries and societies. So a person’s religion may be an accident of their birth (location) or family history. These religions are inherited.
But people do actually adopt religions. Again, this doesn’t necessarily imply acceptance of the belief system. A common reason for adoption of a new religion is marriage. Some religions actually require an intended spouse to accept the relgion and/or agree to raise any children in the relgion. Christopher Hitchens (in God Is Not Great: How Religion Poisons Everything) relates how he had adopted the Greek Orthodox and Jewish religions through marriage.
Social reasons may also be a reason for adherence to a religion. People often require a religious tradition to provide the ceremonies required at the different stages of life. Birth, christenings, circumcision, coming of age, weddings and funerals require ceremonies which religions can provide. The people involved may not accepted the beliefs implied by the ceremonies but feel compelled to undergo them for social reasons. And, of course, there are those who love the tradition and beauty of a ceremony (and its location) despite rejecting the beliefs.
Of course, it is possible to have secular ceremonies for these occasions and, in my opinion, such ceremonies can often be more meaningful than the religious ones because they dispense with the hypocrisy. For example, a non-religious funeral can concentrate more on the memory of the deceased without dilution by dogmatic statements bearing no relationship. However, there seem to be many non-religious people who “put up” with a religious ceremony because they think it would be offensive to family and friends (and society) not to do so.