1 John 3:10

10 This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not God’s child, nor is anyone who does not love their brother and sister.

Read 1 John 3:10 Using Other Translations

In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.
By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother.
So now we can tell who are children of God and who are children of the devil. Anyone who does not live righteously and does not love other believers does not belong to God.