What does John 3:5 mean?

Jesus introduces His comment by twice repeating the word amen. This repetition is frequently translated as "truly, truly," or "verily, verily," or "I assure you," or "I tell you the truth." Amen is an Aramaic word kept intact in Greek, Latin, and many other languages. It literally means, "faithful," or "believe," and is most simply translated as "truly." This is why the term is often used at the end of a prayer or other statement. Putting it at the end of a phrase suggests—or hopes—that what was said is true or sure. Putting the word amen at the beginning of a statement is a way of claiming that this is the speaker's own personal declaration. Jesus is claiming that His statement is absolutely true, and that He has absolute, direct, personal knowledge of its truth.

Jesus makes this comment specifically to Nicodemus, in response to his question about being "born again." There are several possible ways to interpret His comment that only those born of water and of the Spirit can enter the kingdom of God. Being born of the Spirit is clearly a reference to accepting Christ, but the aspect of water is less sure. The most reasonable interpretation is that water refers to the spiritual cleansing which accompanies repentance. This is why John the Baptist's ministry of baptism was a precursor to Christ. In other words, faith in Christ is an absolute requirement for salvation. The physical ritual of baptism is not (John 3:16, 36; Ephesians 2:1–10; Titus 3:5).