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.
If what we have is different than what anyone at any time in history had then God is not eternal, God cannot be trusted to tell the truth for all time. We would have a God that could change anytime God wills it.
I do not believe it is truth that God is a changeable God. I feel the truth is that the law at creation is the very same law we have today, that it took the blood of Jesus for salvation even though man's understanding of that took 4,000 years to be revealed. God knows what is so for all time, God knew of Christ and had ordained it thousands of year before man was told of it.
We are told we see darkly, but what we see of truth has nothing to do with establishing truth. It is true that God is unchangeable and eternal. It is true that never has there been salvation except through Christ. The law as we are told of it in scripture is true law. Man's perception and what man it told of truth has changed as we are led to truth, but our perception is not at all the same as truth.