What did adoption mean in Roman and Greek culture as Paul uses it?

Les explains that in Ephesians 1:5 Paul speaks of believers “having been predestinated unto the adoption of children by Jesus Christ to Himself, according to the good pleasure of His will.” In the Roman and Greek practice, a child born of the husband and wife was put under tutors, educated, trained, and prepared to come in at an equal level with the father, say in a business. When the child reached that point, he went through the rites of adoption. The Hebrew culture had something akin in Bar Mitzvah at age fourteen, but it was not the same category because the Romans and Greeks were in the secular world and were preparing the child to be a co-worker in the business. Les teaches, however, that Paul says the moment we become a believer, we do not go through years of tutoring and training; we are immediately placed as an heir with the Father and the Son. That is what Paul means: believers were predestinated not to heaven or hell, but as believers to this position with Christ from day one, as a joint heir in the process of adoption. Ephesians 1:11 also speaks of obtaining an inheritance as joint heirs with Christ; if you are a joint heir, there are other parties, and in the secular economy if the other half passes off the scene, the heir ends up with it.