Romans 8:15

“For you did not receive a spirit that makes you a slave again to fear, but you received the Spirit of sonship. And by Him we cry, ‘Abba, Father’.”

Paul talks about us being released from slavery to sin and becoming slaves of righteousness (Rom. 6:18). However, when we become a servant of God, He promises us that His yoke is easy and His burden is light (Matt. 11:30). Serving God is not a painful, laborious exercise. He is not a master to be feared. We serve Him out of love (because He first loved us), not because we are afraid of punishment if we fail to live up to His standards – as was the case under the law. Paul reminded Timothy, “God has not given us a spirit of fear, but of power and love and a sound mind” (2 Tim. 1:7).
Rather than being afraid of God any longer, He has adopted us into His family. We, who were alienated from God, are now brought into an inheritance. We receive all the blessings and benefits of belonging to His family, much as if someone was adopted into a royal family today, they would receive the blessings and benefits of it. How much more so in God’s family!
We also have the privilege of calling God our Father. This was a very rare occurrence in the Old Testament (see Ps. 89:26 and Mal. 2:10). Yet in the New Testament, Jesus encourages us to pray, “Our Father in heaven...” (Matt. 6:9). In the Old Testament, people knew God as the creator (Elohim), and as the One who had made the covenant with Abraham, Isaac and Jacob (Jehovah), but they did not know Him personally like we do. It is indeed a privilege for us to be able to call God, ‘Father’.