William Barclay (1907-1978) was a Professor Divinity and Biblical Criticism at Glasgow University in Scotland, and the author of seventeen volumes of commentary on the New Testament, which have an important place on my bookshelf. The following is an excerpt from his commentary on John 3, which I am offering in this forum because it so complementary to the writing of George MacDonald.
In the New Testament, and especially in the Fourth Gospel, there are four closely interrelated ideas. There is the idea of rebirth; there is the idea of the kingdom of heaven, into which people cannot enter unless they are reborn; there is the idea of being children of God; and there is the idea of eternal life...All these ideas have a common thought behind them.
Let us start with the kingdom of heaven. What does it mean? We get our best definition of it from the Lord's Prayer. There are two petitions side by side:
Your kingdom come:
Your will be done on earth as in heaven.
It is characteristic of Jewish style to say things twice, the second way explaining and amplifying the first. Any verse of the Psalms will show us this Jewish habit of what is technically known as parallelism...
Let us apply that principle to these two petitions in the Lord's Prayer...the kingdom of heaven is a society where God's will is as perfectly done on earth as it is in heaven. To be in the kingdom of heaven is therefore to lead a life in which we have willingly submitted everything to the will of God; it is to have arrived at a stage when we perfectly and completely accept the will of God.
Now let us take the idea of being a child of God. In one sense, this is a tremendous privilege. 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 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. S 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 accepeted the will of God.
Now let us take eternal 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 eaven. The idea behind eternal life is the idea of a certain quality of life. What kind? There is only one person who can properly be described by this adjective eternal (aionios), and that one person is God. 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.
Here, then, we have three great related conceptions -- entry into the kingdom of heaven, becoming children of God, and eternal life --- and all are dependent on and are the products of perfect obedience to the will of God. It is just here that the idea of being reborn comes in. It is what links all these three conceptions together. It is quite clear that, as we are and in our own strength, we are quite unable to render to God this perfect obedience; it is only when God's grace enters into us and takes possession of us and changes us that we can give to him the reverence and devotion we ought to give. It is through Jesus Christ that we are reborn; it is when he enters into possession of our hearts and lives that the change comes.
When that happens, we are born of water and the Spirit. There are two thoughts thee. Water is the symbol cleansing. When Jesus takes possession of our lives, when we love him with all our heart, the sins of the past are forgiven and forgotten. The Spirit is the symbol of power. Wen Jesus takes possession of our lives, it is not only that the past is forgotten and forgiven; if that were all, we might well proceed to make the same mess of life all over again; but into life there enters a new power which enable us to be what by ourselves we could never be and to do what by ourselves we could never do. Water and the Spirit stand for the cleansing and the strengthening power of Christ, which wipes out the past and gives victory in the future.
William Barclay, The New Daily Study Bible, Volume One Gospel of John, pages 148-151