How do we know faith is essential in salvation?

In Romans 4, Paul argues that it is through faith that the promises of God are made sure to all of Abraham's spiritual descendants. The law cannot bring about the inheritance; rather, it is faith that allows us to lay hold of the promises given by God. If salvation were based on the works of the law, it would render faith void, as righteousness would be earned rather than given. Therefore, faith is crucial in understanding that salvation is a gift of grace, grounded in the work of Christ rather than our own efforts.