Given a literal reading of this passage, one could use it to continue to separate at will who is 'in' and who is 'out.' But John's purpose is to help the community understand the basics of theology, reiterating that Jesus was both human and divine. Those that denied Christ's divinity are the antichrist because that denial is very antichrist, anti-orthodoxy.
What needs to be reiterated in this passage is the intention that John establishes in naming us as God's children. Children eventually become like their parent. For those who have the claim as children of God live lives that reflect God's life and teaching (righteousness). Those who claim the teaching different from God's have the devil as their parent and do not belong to the Koinonia (the community of faith) will be like their parent. Probably pulling from John's Gospel (John 8:44) the father of lies is the Devil himself.
The positive apporach of this passage is that we are encouraged to live our faith daily, contiunuing to mature in faith and growth as chidlren of God. Thus imitating the righteousness of our Father, our parent, God in Christ."
How do you live out the faith of being God's child?
Lord, move me from my sin into your righteousness so that I may represent you as your child here on earth.-Amen