How do we know that salvation is only through grace?

The truth that salvation is by grace alone is rooted in key biblical passages such as Ephesians 2:8-9, which explicitly state that salvation is a gift from God, not of works, lest any man should boast. This doctrine is critical because it underscores humanity's inability to achieve righteousness on their own, as seen in Romans 3:10 which states there is none righteous, no, not one. The contrast between grace and works illustrates the sovereignty of God's mercy in choosing to save sinners despite their unworthiness, thus highlighting His glory and grace in the work of salvation.