There has to be something to dispensation, because God told Noah to build an arc, does that mean that you should build an arch. You may if you want to, but It wouldn't save you from anything, and of course God wasn't telling you to build an archa because God is not going to flood the earth again. Genesis 9
Also Moses could not have done what Romans 10:9 says because he did not no the name of the Lord Jesus Christ because God had not revealed that to him. He new that God would provide a saviour, even Job said "I know that my redemer livith" Job 27. Also Abrahams bosom is also prof that the old testement saint was not saved as we are now.