When the Jailer in Acts 16:30 asked what must I do to be saved,
And brought them out, and said, Sirs, what must I do to be saved?
He wasnt asking about what qualifying action he do to get himself saved, for he never used the word get as in get saved, but he used the word be, as be saved. The words be saved here are in the passive voice wherein the subject is the recipient of the action, for he was speaking from a frame of helplessness, hopelessness. Pauls response was not that he should do, act, or perform but to believe, that's promoting Faith and not works. Paul knew that believing was the Gift of God, that is, to believe in Christ savingly wasn't a natural inherent quality of the natural man, believing is having been given the Gift of Faith Eph 2:8. Paul testified to the fact that believing on Christ was something God gives here Phil 1:29
29 For unto you it is given in the behalf of Christ, not only to believe on him, but also to suffer for his sake;
That word given is basically the same word for grace charizomai derived from and the middle voice of charis translated 130 times in the NT as Grace.
So people believe because of Grace Acts 18:27.
So now even though technically believing is a doing, which technically is a work, nobody has the ability to believe until God by Grace, by Grace doing, its Gods Grace and Spirit working in the Spirit sanctified sinner causing the sinner to believe. No man by nature can truly believe in Christ as Saviour without first being saved by Grace and given the gift of Faith, so it is written 1 Pet 1:21
21 Who by him do believe in God, that raised him up from the dead, and gave him glory; that your faith and hope might be in God.
Phil 2:13
13 For it is God which worketh in you both to will and to do of his good pleasure.
The do the Jailer needed to do to be saved, God needed to first worketh in him to do.
So thats how Salvation by works and Faith can be scripturally contrasted, when credit is given to Grace for a person doing the believing, and believing wasn't a natural freewill ability they had of themselves prior to regeneration.
But if we insist that believing was a condition we met prior to being saved by grace, and we did it as natural men, and as a result, God saved us, that's making believing, faith a work we do for salvation.