How do we know salvation is by grace according to the Bible?

The Bible asserts that salvation is entirely by grace, particularly illustrated in Romans 4:4-5: 'Now to him that worketh is the reward not reckoned of grace, but of debt. But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness.' This Scripture underscores that if salvation were based on works, it would be a debt owed to humanity rather than a gift from God. God's grace provides salvation freely to those who believe in Him, illustrating the unmerited nature of His favor and mercy toward sinners.