How do we know that the Holy Spirit is necessary for salvation?

Salvation is inherently tied to the work of the Holy Spirit in an individual's life. As stated in John 3:5, Jesus tells Nicodemus that one must be 'born of water and the Spirit' to enter the Kingdom of God. This indicates that regeneration, or being born again, is a work that the Holy Spirit alone accomplishes in a person's heart. Furthermore, Romans 8:9 affirms that anyone who does not have the Spirit of Christ does not belong to Him, emphasizing the necessity of the Holy Spirit for true salvation and relationship with God.