Jesus’ statement in John 3:5—“Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter into the kingdom of God!”—is an amplification of Jesus’ prior statement to Nicodemus in 3:3 concerning the condition for seeing the kingdom of God. The equivalent of being born again or being born from above is literally being born of water and spirit. In the context of John 3:6, 8, where Jesus is arguably talking about the Holy Spirit, it makes sense to translate the phrase born of water and spirit (γεννηθῇ ἐξ ὕδατος καὶ πνεύματος) in John 3:5 as born of water and the Spirit, where Spirit indicates the Holy Spirit.
To be born of water and the Spirit means, therefore, to experience Spiritual regeneration, which ultimately is the work of God. This is the primary idea in John 3:5, but a question remains concerning to extent to which the term water in John 3:5 indicates water baptism. In regard to this issue, the structure of the phrase of water and spirit, where two co-ordinate nouns are governed by a single preposition (i.e., ἐξ), suggests a close connection between water and Spirit. Since Gentile converts to Judaism were considered to become like newborn children through proselyte baptism (which was performed in order to cleanse them from their Gentile impurity), it is quite likely that the word water would have conveyed the idea of baptism, or at least some kind of ceremonial washing, to a Jewish audience, including Nicodemus. Elsewhere in John’s writings where the concepts of spirit and water are placed in close proximity, namely, in 1 John 5:8, spirit refers to the Holy Spirit, and water to Jesus’ baptism.
Jesus’ mention of water and spirit is also to be understood (as it most likely would have been in a Jewish context) in the light of the Old Testament prophecies concerning the outpouring of the Holy Spirit in the new covenant age. The Old Testament prophets foresaw a time when God would work through his Spirit to bring Israel back to himself in covenant obedience (see Deut 30:6; Jer 31:31–33; Ezek 36:24–27). In particular, Ezekiel 36:24–27 pictures the future work of the Spirit as being like water sprinkled upon Israel to cleanse her from her uncleanness. Therefore, understanding the phrase born of water and spirit in John 3:5, in conjunction with the idea of the kingdom of God, on Jesus’ lips, in a Jewish context, leads us to take the phrase born of water and spirit to be referring to conversion or baptism by the Holy Spirit.
But it should be noted at this point that baptism in the Holy Spirit was viewed by the early church as ordinarily taking place at the point of Christian (water) baptism (e.g., Acts 2:38; 1 Cor 12:13; Tit 3:5), following the model of Jesus’ baptism, in which there was a conjunction of water and the Spirit (Luke 3:21–22). Exceptions to the rule of the conjunction of water and the Holy Spirit in baptism only happened at special stages in God’s plan of salvation, such as at Pentecost (Acts 1:15), at the conversion of the Samaritans (Acts 8:14–18), and at the conversion of the first Gentiles (Acts 10:24–48), matching the pattern of the evangelistic mandate in Acts 1:8, where the gospel was to be preached in Judea, in Samaria, and to the ends of the earth (i.e., to the Gentiles). Apart from these exceptions, at least as far as adult converts were concerned, baptism in water and baptism in the Spirit were considered in the early church as taking place together. This led to the view in the early church that the eschatological gift of the Spirit was received through faith at the time of conversion, i.e., at the point of Christian baptism.
It is most likely, therefore, that John’s audience, both Christian and non-Christian, would have understood the phrase of water and spirit in connection with Christian baptism, which marked the point of conversion to Christianity. Conversion to Christianity is the necessary condition for entering the kingdom of God, where entering the kingdom of God is itself a metaphor for coming into the possession of salvation, which involves having the right to live in the presence of God and to experience his blessing. All in all, the significance of Jesus’ teaching in John 3:5 is that Christian conversion, which formally takes place at Christian baptism, which marks the official reception of the eschatological gift of the Holy Spirit on the part of the baptizand, is necessary in order for individuals to experience salvation in the kingdom of God.