The things that @Joshua_Hansen and @SeanO have said are true. God is our heavenly Father by virtue of the new birth.
But sometimes people who claim that we are all God’s children will defend it by claiming that the Bible calls God the Father of all mankind. And there are a couple of passages in the Old Testament that do say that.
Malachi 2:10, have we not all one father? Hath not one God created us?
Isaiah 64:8, O Lord, thou art our father; we are the clay, and thou our potter; and we are all the work of thy hand.
But these verses are meaning that God is the father of all mankind by virtue of creation - as the verses themselves clearly say. In other words, He’s not the heavenly Father of all men - He does not have a spiritual relationship with all men - most men are spiritually dead.
In the same sense, He’s the Father of the angelic hosts by virtue of creation. Job 38:4-7 says that when God laid the foundations of the earth, the morning stars sang together, and all the sons of God shouted for joy. Obviously these cannot be humans shouting for joy when God lays the foundations of the earth on Day One.
But Hebrews 1:5 says that God never called any angel His Son the way He did with Christ. The relationship that the only begotten Son of God has with the heavenly Father is unique.
And yet, He shares it with all men who are begotten of Him at the moment of salvation.
So all men are His natural children by virtue of their natural birth into His creation. But this creation will pass away, and those who are only connected to Him through it will pass away with it.
But Christ’s followers are his spiritual children by virtue of their spiritual birth into His new creation that will last for one day - but it will be a very long Day (Revelation 22:5)!