Those under the OT were kept safe, not eternally saved like you and I, by following the law. If they did not walk in the obedience of the law they were cut off from God’s people.
Before the law, man was responsible for obeying whatever it was that God gave them at that time to follow.
Abraham is in type an example of believing God’s word and receiving righteousness but he is not identical to what we have. The message has changed. Also, his justification came by sacrificing Isaac. Our justification happens the moment we believe the gospel of Jesus Christ.