To those who believe, there is given the power to become God's children (John 1:12). But the very essence of being a child of God is necessarily obedience. They who have My commandments and keep them are those who love Me" (John 14:21). The essence of this relationship is love; and the essence of love is obedience. We cannot with any reality say that we love a person and then do things which hurt and grieve that person's heart. This relationship is a privilege, but a privilege which is entered into only when full obedience is given. So then to be children of God and to be in the kingdom are one and the same thing. The children of God and the citizens of the kingdom are both people who have completely and willingly accepted the will of God.
Now let us take eternal life. It is far better to speak of eternal life than to speak of everlasting life. The main idea behind eternal life is not simply that of duration. It is quite clear that a life which went on forever could just as easily be hell as heaven. The idea behind eternal life is the idea of a certain quality of life. What kind? Eternal life is the kind of life that God lives; it is God's life. To enter into eternal life is to enter into possession of that kind of life which is the life of God. It is to be lifted up above merely human, transient things into that joy and peace which belong only to God. Clearly we can enter into this close fellowship with God only when we render to Him that love, that reverence, that devotion and that obedience which truly bring us into fellowship with Him.
Finally, in this passage, John lays down a great law. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. Human beings by themselves are flesh, and their power is limited to what the flesh can do. By themselves, they cannot be other than defeated and frustrated; that we know only too well; it is the universal fact of human experience. But the very essence of the Spirit is power and life which are beyond human power and human life; and when the Spirit takes possession of us, the defeated life of human nature becomes the victorious life of God.