1. Because Paul said to believe on Christ is how one is saved today, and also
2. Paul said that salvation is not of works.
Ergo, believing is not, and cannot be, a work.
Really simple when you set aside your agenda and just believe what it says.
So "believing" isn't a work , even though its done with an expectation of something in return (thus being paid for your work). But surrendering our lives to Him "is' a work even though if done correctly leaves no life available to recieve any expectation of payment .
And i won't even get into the millions of people before the 1500's that could never be exposed to "Gods Word to believe in".
What saves us with Gods Word should have been able to save without Gods Word too.
Believing isn't it surrendering is.