How do we know that regeneration is essential for Christians?

John 3:5 explicitly states that 'except a man be born of water and of the Spirit, he cannot enter the kingdom of God.' This underscores the doctrine of total depravity, where man, in his natural state, cannot achieve righteousness or salvation. The necessity of being born again is reinforced by the understanding that our first birth does not equip us spiritually. The Holy Spirit's work is vital in opening our eyes to truth and imparting new life, indicating that regeneration is not merely beneficial but absolutely essential for salvation and eternal life.