How do we know that salvation is by God's grace alone?

Salvation is portrayed in Scripture as a gift from God, highlighting that it is by grace alone. In Ephesians 2:8-9, Paul states, 'For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast.' This clearly indicates that no amount of human righteousness can earn salvation, as our best works are described as 'filthy rags' in Isaiah 64:6. Instead, God has chosen to save His people according to His mercy and grace, as seen in Titus 3:5, where we learn that we are saved, not by our own righteousness, but according to His mercy.