“For in Christ Jesus you are all sons of God, through faith.” (Galatians 3:26)
Since the Lord God created mankind, all humans were His “children”. Sadly, we turned our back on Him and broke the relationship. But the salvation work of Jesus Christ, the Son of God, enables every human who believes in Him to become a son or daughter of God again. As John 1:12 says: “To all who did receive Him [=Jesus], who believed in His name, He gave the right to become children of God”.
In Old Testament times, God had a special relationship with Israel, His “firstborn”. People from other nations could become part of God’s people by being incorporated in Israel. But since Jesus has come, you no longer need to become a Jew in order to be a child of God. No, you will be part of “a great multitude that no one [can] number, from every nation, from all tribes and peoples and languages” (Revelation 7:9).
The only thing needed to become a child of God, is faith in Jesus. For He has provided everything necessary for you to have your sins forgiven, to be reconciled with God and to have the relationship between Father and son/daughter restored.
Are you already a son or daughter of God? He invites you into His family!