Romans 8:15 (ESV): 15 For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!”
Having received the indwelling of the Spirit of God, believers not only take on his communicable attributes—God likeness (godliness) as sons—believers are called to have intimacy with God. Two stations of intimacy are contrasted here: slaves and sons.
Slaves relate to their master in terms of always fearing to displease and receive chastisement for displeasing him. There is not an inheritance or consideration for an inheritance for a slave. Sons, on the other hand, can cry Abba. Legally, adopted sons have all the rights of a biological son but can never be disowned.