New Defender's Study Bible Notes
3:3 born again. The vital doctrine of regeneration, or the new birth, has been applicable in all ages, for man by nature is a lost sinner, and must be reborn spiritually through faith in God and His promises if he is to be saved. Note, for example, such Old Testament Scriptures as Isaiah 1:18; Psalm 51:10; Ezekiel 18:31. Nevertheless, this vital doctrine is crystallized and clarified and individualized more in the New Testament, especially in this chapter. See also II Corinthians 5:17; Galatians 6:15; Colossians 3:10; Titus 3:5; I Peter 1:23; and other New Testament verses on the new birth.
3:3 kingdom of God. Because of the preaching of John the Baptist that “the kingdom of God is at hand” (e.g., Mark 1:15), Nicodemus had undoubtedly been studying the Biblical promises of the kingdom—perhaps such passages as Isaiah 9:6-7; Zechariah 14:9; Daniel 7:13-14; Psalm 72:1, 7-11; as well as others. But now he is surprised to hear Jesus say that one cannot even see that kingdom without being born again!