I pray also that the eyes of your heart may be enlightened in
order that you may know the hope to which he has called
you, the riches of his glorious inheritance in the saints,
Jerry Bridges, in his book Who Am I?, tells us the difference between adoption in Bible times compared to the way it is today:
Adoption in Paul's day was completely different from our practice today. In our modern culture, adoption usually refers to the adoption of an infant or small child. In the Greco-Roman culture, a childless man would adopt a mature young man to continue the family name and to receive the family inheritance. In the Jewish culture, adoption was the recognition that a son had reached an age of maturity whereby the authority and resources of the father would be passed on to him. In either case, it was the family inheritance that was in view.
Keep those differences in mind while we talk about being adopted into God's family. Let's look at two passages:
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." The Spirit himself testifies with our spirit that we
are God's children. Now if we are children, then we are heirs--
heirs of God and co-heirs with Christ, if we indeed share in his sufferings in order that we may also share in his glory.
You are all sons of God through faith in Christ Jesus...There is
neither Jew nor Greek, slave nor free, male nor female, for you
are all one in Christ Jesus...What I am saying is that as long as
the heir is a child, he is no different from a slave, although he
owns the whole estate. He is subject to guardians and trustees
until the time set by his father...So you are no longer a slave, but
a son; and since you are a son, God has made you also an heir.
Galatians 3:26, 28; 4:1-2, 7
Why does Paul call believers "sons," if adoption refers to both male and female? Listen again to what Jerry Bridges says:
Then why the word sons? Because Paul was placing a priority on communicating clearly to his immediate audience. In the Jewish culture of the day, only males were eligible to receive a part of the family inheritance. So, far from putting down women or ignoring them, Paul is actually making them equal with men in sharing the family inheritance. We are all one in Christ Jesus, spiritual offspring of Abraham, and equally heirs of God's promise.
Being a son has to do with being an heir, but today we would say we are an adopted child of God because inheritance can be for all our children, whether male or female. In the spiritual sense, our adoption has to do with both a future and present inheritance.
OUR FUTURE INHERITANCE
Anything we suffer now is nothing compared to our future glory. The Holy Spirit is given to us as a foretaste of this future glory. But for now, we must wait with hope for our bodies to be released from sin and suffering. We must wait patiently and confidently for the day when God will give us our full rights as his adopted children, including the new bodies we are promised.
Hope in this context means waiting with expectancy. Paul is contrasting our future inheritance with our present suffering. Sometimes I think Christians fall into the trap of thinking something is wrong is they are suffering, but that isn't what Scripture teaches. Romans 8:17 says you will share in Christ's suffering so that you may also share in His glory.
In short, our future inheritance involves heaven and all the promises that go along with being in heaven. In this life, believers live in the tension between the "already" of redemption and the 'not yet" of the glory to be revealed. We live with the frustration of never being entirely victorious over sin as well as its consequences. We experience sickness, injury, depression, unmet needs, and unfulfilled desires. But we have a present inheritance too that will help us as we wait. In this life, there is a benefit to being an heir.
OUR PRESENT INHERITANCE
In both Romans 8:15 and Galatians 4:6, Paul says as a child of God, we call Him "Abba Father." Abba is an Aramaic word used primarily by Jewish children to address their fathers. It is a term of endearment and implies a child-like dependency and an expectation that their Abba will meet their needs. It is also how Jesus addressed His Father in the Garden of Gethsemane. Today, in this country, a child might say Daddy.
We get to address the sovereign Creator of the universe as Daddy. We, who are dependent on and accountable to God, can come to Him as our Abba. Furthermore, we can come anytime. In Old Testament times, only the High Priest could enter the Holy Place behind the curtain, where God "dwelled" and only once a year on the Day of Atonement. When Jesus died on the cross, the curtain was literally torn from top to bottom by God (Mt 27:51). And now all adopted children of God have access to Him. Hebrews 4:16 says, "Let us then approach the throne of grace with confidence, so that we may receive mercy and find grace to help us in our time of need."
In addition to receiving mercy and finding grace in our time of need, we will receive wisdom when we ask God for it without doubting (Jms 1:5-8). We will receive rest when we are weary and burdened if we give our concerns to Him (Mt 11:28-30). We will receive the peace that passes understanding when we, by prayer and petition, with thanksgiving, present our requests to God instead of living in worry and anxiety (Phil 4:6-7). We will receive forgiveness when we confess our sins (1 Jn 1:9). And we will receive joy when we take the time to praise, rejoice, and be thankful; when we put your focus on our Abba Father (Ps 100:4).
All of these actions, asking, giving, praising, and rejoicing, can be done symbolically at the throne of God. Any time you pray, you are at the throne. When you sing worship songs, you are at the throne. And all this is possible because you have been adopted by God into His family!
Source: Jerry Bridges: Who Am I? Identity in Christ