“The Spirit of sonship”
For you did not receive a spirit of slavery that returns you to fear, but you received the Spirit of sonship, by whom we cry, 'Abba! Father!' (Romans 8:15 BSB)
What it sounds like today
'Spirit of sonship' sounds like a soft, vague spiritual status.
The cultural key
The Greek behind 'the Spirit of sonship' is huiothesia, literally 'adoption as a son.' Roman adoption was chiefly a legal instrument for adults: a man without a suitable heir adopted a grown son, often to continue his name and estate, and the act was total. The adoptee's old debts and obligations were extinguished, his old paternity legally ended, and his new sonship carried full inheritance rights; the emperors themselves ran on it (Augustus was Julius Caesar's adopted son, Tiberius was Augustus'). Rome's readers knew adopted sonship as, if anything, more deliberate and more secure than birth. And the cry of the adopted is Abba, the Aramaic family word from Jesus' own praying (see the Mark 14:36 entry).
Recovered meaning
Not sentiment but legal transfer: old master's claims void, new Father's estate entailed, and the Spirit as the family resemblance already speaking. The empire's most powerful legal fiction becomes Paul's picture of grace, chosen, total, and irreversible.