According to the Bible, God gave his law to his people through Moses. The only time there was anything one might refer to as "new light" was when Jesus came to earth to fulfill the law. That's it.
So, for centuries the Israelites had the Mosaic law with no new light. Then Jesus begins his ministry around 30AD. We get the Sermon on the Mount and a few other public discourses sprinkled with a few private meetings with the disciples. That's it.
Even decades after Jesus' death and resurrection, the writers of the New Testament refine nothing. No new light. No change in Jesus' teachings. No "clearer understanding" of the Gospel.
If anyone knows how many times Watchtower claims to have received "new light" I would like to know that number. I'm sure it will be a sharp contrast to the one time it actually happened and it took Jesus coming to Earth to do it.