How do we know that salvation is by the Spirit?

According to Ephesians 2, we were dead in our sins until God made us alive through His Spirit. It is a work of God’s grace where He imparts spiritual life to us, enabling us to believe and receive the gospel. Romans 8:9 clarifies that anyone who does not possess the Spirit of Christ is not His. The Spirit is essential for understanding divine truths and for experiencing true faith; hence, without Him, we cannot attain salvation. This affirms the Reformed doctrine that salvation is wholly an act of God's sovereign grace, dependent on His Spirit rather than our works.