How do we know that spiritual regeneration is necessary?

According to Ephesians 2:1, those who are natural men are 'dead in trespasses and sins', signifying a complete inability to respond to God without being made alive by the Holy Spirit. This regeneration transforms one's heart, allowing them to embrace the gospel and comprehend the fullness and freeness of salvation in Christ. When a person is spiritually regenerated, they are not just revived; they are endowed with the ability to discern and appreciate the things of the Spirit, as seen in 1 Corinthians 2:15, where Paul says that the spiritual man discerns all things that are freely given by God.