How do we know that grace alone is the foundation of salvation?

Romans 11:5 states, 'Even so then at this present time also there is a remnant according to the election of grace. And if by grace, then it is no more of works; otherwise grace is no more grace.' This passage clearly sets a distinction between grace and works, asserting that if salvation includes works, it is no longer solely grace. Thus, grace alone is fundamental to salvation, as it emphasizes that God, in His mercy, unilaterally declares sinners righteous on account of Christ's finished work, without any merit of their own.