Is everyone a child of God?
We are all the offspring of God in the sense we are His products of the creative work of God (Acts 17:25ff), all men are not the children of God in a personal sense as a father with a son. Scripture teaches that God only dwells in those who have put their faith in Christ. All others, while God’s offspring by creation, have been separated from Him because of sin. When a person turns to God by believing in His Son, Jesus Christ, they are spiritually regenerated, born anew, given new life, justified (declared righteous in Christ), forgiven of all their sin, past, present, and even future, and made children of God (John 1:12-13). At the same time, at the moment of faith, God the Holy Spirit comes to indwell their lives to enable them to experience fellowship with God and understand, grasp the truth of Scripture in a personal and meaningful way.
For the details of this, may I encourage you to check out the Part 1 of, Laying the Foundation: ABCs for Christian Growth. The first section explains the gospel message. It explains why people are lost without Christ and how they may come to Christ and experience God’s forgiveness and salvation.