Men have always been saved by grace through faith, but God has not always required the same expression of faith from us.
God created Adam and Eve and placed them in a garden in order that He could share a relationship with them. That relationship was based upon freedom. So Adam and Eve were free to go anywhere and do anything. The one commandment God gave them was that they not eat of His tree. In the garden the relationship with God was maintained by adherence to one commandment. Their faith was expressed by not eating.
After the fall men lived without much addition to the one (now redundant) commandment. For nearly 2,000 years men lived as they saw fit and almost to a man they turned away from God. Relationship with God was based upon the willingness of people to seek God entirely of their own volition. Their faith was expressed by their own standards.
After the flood God instituted laws of governance for murder and some dietary laws. He set up a covenant with the promise that He would never again destroy the earth with water. Relationship with God was based upon respect and remembrance of His capacity to act upon His word. At this time faith was expressed by respect for God.
To Abraham God gave a promise and Abraham believed that promise. His relationship with God was sealed because he believed His word. His faith was expressed by his belief.
To Moses God gave the law. The nation of Israel had relationship with God through the law and the nations of the world had relationship through Israel. Israel's faith was expressed by their adherence to the law.
There are five different time periods where God entered into relationship with people under distinct conditions. Four times the conditions changed either explicitly or implicitly. You can imagine the confusion or conniving of people who might live at the time of one of the changes. Imagine had Cain demanded that he be allowed the same conditions as his parents. What do you think would have happened had some Israelites ignored the law and demanded a relationship like God had with Abraham.
And compare this to what we know today. We are not in relationship with God because we do not eat. We have not approached God of our own volition. We might respect God's ability to act on His word, but that doesn't save us. We can believe in God, but even demons do that! We can follow the law but believing that any part of the law is obligatory puts us at the mercy of the entire law.
Mike - Do you agree that throughout history God has changed the rules about how we are to express our faith?