“So you have not received a spirit that makes you fearful slaves. Instead, you received God’s Spirit when he adopted you as his own children.^[a] Now we call him, “Abba, Father.” For His Spirit joins with our spirit to affirm that we are God’s children. Romans 8:15 -16 NLT 

In the Roman world of the 1st century, an adopted son was deliberately chosen (often as an adult) by his adoptive father to perpetuate his name and estates. (That’s how Nero became emperor.) As such, society considered him to be equal with natural sons, and he might actually enjoy his father’s affection even more than they did. 

It was an honour to be adopted into an important family and for Christians to be adopted into the family of God there can be no greater honour than to perpetuate the values and name of the Father. The whole process of adoption moves one away from fear and from bondage to sin and toward the freedom of sons and daughters. If sin and its entanglements lead to fear, the apostle John let us know that adoption into God’s family frees us from fear. “There is no fear in love; but perfect love casts out all fear” (1 John 4:18). 

Being adopted into the family of God means that we need not fear the Father. In fact, we may address Him in not only the more formal manner as Father but also as “Abba”. Abba is a more everyday word. Ordinarily when Jews called Him Father they quickly added “in heaven” to demonstrate God’s transcendence and distance from them. 

We need to remember that the father in a Roman household was an awe-inspiring person who quite literally had the right to put family members to death. Thus even though the title Abba suggests love and intimacy, it is still a title of reverence. 

Jesus addressed God as “Abba, Father” in Gethsemane. And in v15 we find that we have the same privilege. The term itself reflects God’s closeness to each of us. He is not somewhere out there, but with us and willing to help us in our time of need.  

V16 seems to add a contrast as we contemplate the subjective presence in our lives that seems to confirm our adoption into God’s family – the word “know” occurs 44 times in the 5 chapters of John’s first epistle. He concludes that we can “know that we have eternal life” 1John5:13. This is the Holy Spirit bearing witness with our spirit. We know we are His child.