As far as dispensations go, they are not about how salvation works. There is no other name than Jesus Christ, given among men, whereby we must be saved.
In the NT, the just shall live by faith. In the OT, under the Law, and it is referred to in the NT, the just live by faith under the Law.
In the OT, before the Law, Abel, Enoch, Noah, Abraham, Sarah, Lot, Isaac, Jacob, Joseph, and others all lived by faith.
Dispensations is not about different ways to be saved. It merely shows how God worked His plan for the salvation of mankind. God is not still penning Scripture. God is not speaking directly in conversation as to Cain or Noah or Enoch.
Clearly we live with a different relationship to God than Adam in the Garden, man under self government and direct contact with God, man under the governments of men as prescribed by God, Israel under the Law of God, and the world under the rule of Jesus Christ from Jerusalem.
None of these change that the “just shall live by faith.”
I am always wary of the question of dispensations because it is just like the Calvinism discussion. Not everyone takes it as far as some people do.