on the particle de (1161)
In John 3:1, RV, it may not at first sight seem clear whether the de, "Now," is copulative, introducing an illustration of Christ's absolute knowledge, or adversative, signifying "But." In the former case the significance would be that, however fair the exterior might be, as exemplified in Nicodemus, he needs to be born again. In the latter case it introduces a contrast, in regard to Nicodemus, to what has just been stated, that "Jesus did not trust Himself" (2:24) to those mentioned in v. 23. And, inasmuch as He certainly did afford to Nicodemus the opportunity of learning the truths of the New Birth and the kingdom of God, as a result of which he became a disciple ("secret" though he was), he may be introduced in the apostle's narrative as an exception to those who believed simply through seeing the signs accomplished by the Lord (2:23).
- from page 695, Vine's Complete Expository Dictionary