Possession is believed to be the result of having made a vow to the wrong party, or to have become a member of an illegitimate belief. It's related legally to the direction not to be unequally yoked together with unbelievers, presently thought to apply only to marriage (I wonder how the recently legal homosexual married couples are getting along with that). According to classical theology, God recognized and true oath that you knowingly and willingly swore in His name under the Second Commandment. If you were tricked into joining a cult by people who said that they were Christians and inducted you, by baptism for example, or swearing in public or by contract in a business arrangement with a non profit, then you meet the legal definition of "possessed".