Is everyone a 'child of God'?
Despite what is commonly held or repeated, the household of faith are the only children of God as only true believers in Christ are His family.
Romans 5:10 tells us that we used to be enemies of God, Colossians 3:6 speaks of the wrath of God that comes on the 'children of disobedience', Ephesians 2:3 speaks of us Christians who were once 'by nature the children of wrath'.
Romans 9:8 tells us that not all the natural seed (offspring) of Abraham are the 'children of God', but instead 'the children of the promise are counted for the seed' (see Galatians 3:8, 14 and 29 for how both Jews and Gentiles are only made this through faith alone). 1 John 3:10 is crystal clear on this matter, when it speaks of knowing the children of God from the 'children of the devil'.
So, this nonsense of everyone being children of God is wrong according to the Word of God, and in Galatians 4:5-6, Ephesians 1:5, John 1:12-13, John 8:40-59, Hebrews 12 and Romans 8:14-17 it speaks of adoption, which is an act of God's free grace where we are received into the number, and have a right to all the privileges of the sons of God.