What does it mean to be a child of God?
"The Spirit Himself bears witness with our spirit that we are children of God, and if children, then heirs--heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together" (Romans 8:16-17).
Jesus told Nicodemus that entering the kingdom required being born again: "Jesus answered and said to him, "Most assuredly, I say to you, unless one is born again, he cannot see the kingdom of God" " (John 3:3). When a person is newly born, he is a child. Thus, when a person becomes a Christian he is born again, becoming a child of the one who gave him birth -- God. Again, Jesus told his disciples, "Assuredly, I say to you, unless you are converted and become as little children, you will by no means enter the kingdom of heaven" (Matthew 18:3).
All Christians, therefore, are in the family of God where God is the Father and we are each his child. "Now, therefore, you are no longer strangers and foreigners, but fellow citizens with the saints and members of the household of God" (Ephesians 2:19). Of course, in a family it is expected that the children be obedient to their parents. It is no different in God's household. "Therefore gird up the loins of your mind, be sober, and rest your hope fully upon the grace that is to be brought to you at the revelation of Jesus Christ; as obedient children, not conforming yourselves to the former lusts, as in your ignorance; but as He who called you is holy, you also be holy in all your conduct, because it is written, "Be holy, for I am holy."" (I Peter 1:13-16). And as children, we face discipline from our Father. "And you have forgotten the exhortation which speaks to you as to sons: "My son, do not despise the chastening of the LORD, nor be discouraged when you are rebuked by Him; for whom the LORD loves He chastens, and scourges every son whom He receives." If you endure chastening, God deals with you as with sons; for what son is there whom a father does not chasten? But if you are without chastening, of which all have become partakers, then you are illegitimate and not sons. Furthermore, we have had human fathers who corrected us, and we paid them respect. Shall we not much more readily be in subjection to the Father of spirits and live? For they indeed for a few days chastened us as seemed best to them, but He for our profit, that we may be partakers of His holiness. Now no chastening seems to be joyful for the present, but painful; nevertheless, afterward it yields the peaceable fruit of righteousness to those who have been trained by it. Therefore strengthen the hands which hang down, and the feeble knees, and make straight paths for your feet, so that what is lame may not be dislocated, but rather be healed" (Hebrews 12:5-13).
Being a child in a family means we can expect an inheritance from our parents. This too is paralleled in the Christian's relationship to God. "But when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law, to redeem those who were under the law, that we might receive the adoption as sons. And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, "Abba, Father!" Therefore you are no longer a slave but a son, and if a son, then an heir of God through Christ" (Galatians 4:4-7).