Those who are saved are God's own, deserving of love. Others are not God's children. God loves all beings and all are his children. It is possible to judge others, or at least to know how God will judge them in spiritual terms. I cannot judge another in terms of their relationship to God, for I have quite a bit to do to determine what God wishes for me to do.
Judgment is more self-inflicted.
Knowing God God can be known through his word and commandments. If you have knowledge of the Law, you know God. God is essentially unknowable by knowledge, but we can know something of God by grace, loving God, and loving each other in the mystic body of Christ. Satan is a creature who deceives, tempts and puts into bondage, misleading some into thinking they are Christians acting from God. Darkness is separation from God.