How do we know that the new birth is necessary for salvation?

In John 3:3, Jesus told Nicodemus that unless one is born again, he cannot see the kingdom of God. This new birth is a divine act of God, where He transforms the heart of a believer. It is not a result of human effort or decision, but rather an initiative of God's grace. This spiritual birth signifies a change of nature; just as physical birth is something one cannot accomplish on their own, so is spiritual birth. The necessity of the new birth underscores the sovereign grace of God in salvation, affirming that it is He who brings about this life-altering experience.