Paul thought Jesus and God had selected people to be “adopted” before the world had been created. These people would “inherit” in the kingdom of heaven through God and Jesus’ grace.
Adoption and inheritance were related in Paul’s mind by the fact that they were both spiritual matters. The adoption was an adoption into the church, and the inheritance was that of the kingdom of god. Both of these are matters of the heart and have to do with salvation. You are not adopted into the Lord’s church without salvation, nor do you inherit the blessings that come along with it. You need one in order to have the other.
By receiving the spirit into his heart and placing his faith in god, one is adopted into a spiritual family and gains access to the inheritance that is heaven. Without this system, there would be no way for one to gain forgiveness and receive the inheritance of god.
Picture Credit: JesusWalk