In response to Nicodemus's question about being born again, Jesus makes this specific comment. There is a reasonable interpretation that water refers to baptism, but in the context of repentance, faith in Christ is the prerequisite to salvation. This is why John the Baptist's ministry of baptism was considered a precursor to the ministry of Jesus. 
Although Nicodemus was a teacher of the Law, he lacked understanding, and Jesus clarified, emphatically and clearly, that unless one is born of water along with being born of the Spirit, they cannot enter the kingdom of God. As a result of trusting in Christ for salvation, a person is believed to be born of the Spirit. 
As with all of us, Nicodemus had to learn that God loved the world so much that He gave His One and Only Son so that whoever believes in Christ will not perish but will have eternal life. As we must do, Nicodemus came to see that the world will be saved through God's grace and faith in Him. In order to live, both Jews and Gentiles must look to Christ.

Dear Heavenly Father, thank You for giving me life and for everything it means to be human. More than that, though, I am grateful that Your Holy Spirit made me conscious of my sin, of Christ's righteousness, and of the terrible condemnation that awaits all who refuse to believe in Christ for the forgiveness of sins and eternal life. It is through Christ's precious blood that I have been born again and cleansed. 
In Jesus's name I pray. Amen.