How could one be saved by Old covenant law?
moses gave them the ultimatum of how they could be saved,
they had to not only confirm, but obey every letter, failure to do so brought forth a curse, (see Gal 3 with duet 27: 26)
which meant if a person sinned even even one time, they were doomed forever
that is why God introduced the sacrificial system as a tool to show, no one can fulfill the law and save themselves, for that reason, an inocent had to die to redeem or atone for your sins, because you can not save yourself you are lost under just one sin and no one is sinless,
jesus became that lamb, the redeemer Abraham David and job understood about the redeemer the prophets prophesied about, the lamb of God who would pay for the sins of the world, all te sin from Adam until the last man.
It is troubling that that people think anyone is saved by OT law, it is sadly telling that the same people do not believe in eternal security.