In ancient Rome, the father of the house had absolute power, known as "patria potestas". He had the power to put full grown sons to death, on a whim, if he so chose. His sons could hold office, be married, and have children of their own, yet all true power resided in the father.
The father also had the power of adoption. The Roman concept of adoption was not limited to infants or small children, as ours (usually) is. At times, full grown men would be adopted into a powerful family to strengthen ties with another family. The father even had the power to free a slave, then adopt him into his family, and under the law, that slave would have the same rights (including citizenship) that any natural-born child of that father would have. I doubt that that would happen very often, but it is an illustration of the "patria potestas".
Now, the adopted person was looked upon as being so much a son of his adopted father, that any debts that his old family had could not be brought against him. He was looked upon in all respects as a true son of the adoptive father.
This is not just an illustration of our relationship with God, the Bible says that this is our relationship with God, our heavenly Father, by whom we are made co-heirs with Christ in all of His blessings. Praise God! Satan has nothing to hold against us, no IOU that he can wave in God's face demanding payment, because we are part of a new family.
I am so happy to be part of my new family, and I love the fact that I have millions of brothers and sisters, that we have all been chosen by God, picked out before we ever were.