Verses 3:1-8 contain the key message of this Gospel. As Rudolf Bultmann states on pp. 133-142 of The Gospel of John: A Commentary

The Mystery of Rebirth: [Chapter 3]

"...it is hardly probable that Nicodemus is intended to represent one of the unreliable "believers." Jesus' miracles have, it is true, made an impression on him also, but this does not mean that they have moved him to "faith;" they have drawn his attention to Jesus and set him asking questions. Nicodemus himself is otherwise unknown. The description given of him is important, for in him Jesus is confronted by a representative of official Judaism. He is a Pharisee and a ruler and, as is suggested, a scribe. This then, is the kind of man on whom Jesus' appearance has made an impression, and it comes to him by night.

The words which he addresses to Jesus have the form of a simple statement, but in fact they contain a question, as shown by Jesus' reply. It would be wrong to give a psychological or individual interpretation of this question. Nicodemus comes with the one question which Judaism, of which he is a "teacher," has to put to Jesus, and must put to him. It is the question of salvation (soteriology.) Jesus replies by giving the conditions for entry into the rule of God, and in doing so he proceeds on the self-evident assumption that for the Jews the question of salvation is identical with the question of participation in the rule of God. It is typical that Nicodemus should use the form of an indirect question, starting from what is safe ground. This much one could say with certainty on the basis of Jesus' miracles; he is an accredited "teacher"--for this is the only title which the Scribes could have given him--and Nicodemus accordingly has addressed him as "Rabbi."

If Nicodemus' question has been based on an understanding of the matter formed in accordance with traditional standards of judgement, Jesus' answer tears him away from his rational considerations and makes him listen to something which he cannot understand. He may well consider Jesus, a man accredited by miracles, to be a superior teacher to him; the Revealer, however, is not quantitatively, but qualitatively superior to the human teacher, and the criteria which the latter has at his disposal are not capable of providing an adequate basis for understanding him.

The reply in GosJohn 3:3 reads: "Unless a man be born anew, he cannot see God's rule." Thus right at the beginning it is stated uncompromisingly that man, as he is, is excluded from salvation, from the sphere of God; for man as he is, there is no possibility of it. Yet at the same time it is said in such a way that a hint is given that salvation may be a possibility for him, inasmuch as it is possible for him to become another man, a new man. The saying therefore also contains an injunction; not, however, of a moralistic sort, but rather the injunction to put oneself in question. This is emphasized and made clear by Nicodemus' misunderstanding in 3:4; for the Evangelist chooses this grotesque way of making it abundantly clear that rebirth is in no sense a natural process, an event which can be set in motion by man himself. In the human sphere it is impossible for there to be anything like a rebirth. For rebirth means--and this is precisely the point made by Nicodemus' misunderstanding--something more than an improvement in man; it means that man receives a new origin, and this is manifestly something which he cannot give himself. For everything which it lies within his power to do is determined from the start by his old origin, which was the point of departure for his present life, and by the person he has always been. For it is one of the basic ideas of Johannine anthropology--as was hinted in the Johannine Prologue (see later Section)--that man is determined by his origin, and determined in such a way that, as he now is, he has no control over his life, and that he cannot procure his salvation for himself, in the way that he is able to procure the things in this life. Moreover the goal of man's life corresponds to his origin. If his way is to lead to salvation, it must start from another point, and man must be able to reverse his origin, and to exchange the old origin for a new one. He must be "reborn!" [Note the strong resemblance to ApJohn's Autogenes Process/Spirit of Light.]

Nicodemus cannot see that there is such a possibility, and Jesus' reply in Verse 3:5, which is made with great emphasis, repeats again the condition for participation in salvation; and yet he repeats it in a slightly different form, which suggests an answer to the riddle. In the first place this means that the condition can only be satisfied by a miracle since the root word refers to the power of a miraculous event. Secondly, however, it suggests to Nicodemus, and indeed to anyone who is prepared to entertain the possibility of the occurrence of a miraculous event, that such a miracle can come to pass...man is ultimately a stranger to his fate and to his own acts; that is, as he now is, he does not enjoy authentic existence, whether he makes himself aware of the fact of whether he conceals it from himself. The miracle of a mode of being in which man enjoys authentic existence, in which he understands himself and knows that he is no longer threatened by nothingness, [represents this state.]

Man stands, as it were, between these two possibilities of existence, in that he knows, or is able to know, that his proper place is in the other-worldly being, whereas in fact he has become entangled in the this-worldly. The statement in 3:6 is intended to make the man, who, like Nicodemus, is searching for salvation realize that these two possibilities of being are not open possibilities between which he can freely choose; that the alternatives by which man is confronted are not governed by choice, but by destiny. It is intended to make him aware that his goal is determined by his origin, and that his end will be nothingness, because it is nothingness that he has his origin. He must realize that everything that he can achieve, and everything that can happen within the sphere of life in which he has existed till now, will come to nothing. Nor will the sphere of this worldly, human life provide him with the miracle he is looking for. If he is to enjoy a miracle at all, his whole being from its very origin must be changed into a miraculous, other-worldly being. Once he has understood this, he will no longer be surprised by talk of rebirth (3:7.) For he will then understand that the attainment of authentic existence can for him be only a miracle--just as the "becoming like little children" which, according to the Dominical saying in the Synoptics, is the condition of salvation, cannot be achieved by deliberate action on the part of man, but can only be received by him as a divine gift."