Yep they have to believe God
who told them they had to confirm and obey every word, if they failed they were cursed (in other words, Gods demand was perfection)
for those who were not perfect (everyone, for all have sinned) there was the sacrificial system, which showed what had to happen for mankind to even come near god,
it also showed mankind’s position was so severe only a high priest could come to Gods presence, and even at that, only after the was cleansed. and if he failed in even one are, he died instantly.
the law NEVER saved on person, it condemned every person, because NO ONE could keep the law as required by God, that’s WHY God had to send his son, it’s also why God had to “pass over” the sins of the OT saints, because although they were saved by grace through faith, their sin debt had not been paid yet,