All human beings are children of God as long as we remain obedient to God. Man lost the privilege of being called the children of God when he also obeyed the law by doing the will of God that man will be qualified to be called children of God.
In Paul's letter to the Galatians he assured them that they were children of God because Jesus Christ the author and finisher of their faith is the son of God. Through the Galatians Christians Paul reached out to all Christians on how to be children of God. He said there should be no discrimination among Christians since those baptised have put on Christ.
He said, all Christians are one in christ Jesus if anyone is in Christ, then he is Abraham's offspring according to the promise. When men were children spiritually, there were also slaves to the elemental/spirits of the universe. But God sent his son so that men might receive adoption as sons when they became sons, God sent his son's spirit into men's heart carrying "abbal father".
Therefore through God believers or Christians are no longer slaves but sons and heirs. Thee are ways we can show that we are children of God:
1.) We should reject Satan in our lives and accept Christ as our saviour.
2.) We must obey and fulfill christian obligations and those of state also.
3.) We must strive hard to win more souls for Christ and contribute our quota to the propagation of the gospel and Christianity.
As Paul said that Christians are all children of God by faith we should endeavour to live mature in faith by living exemplary life according to the teaching of Christ. It is maturity in faith that would qualify Christians to call themselves Children of God and also call God their father and he would answer them.