And I really wonder why there was such a long wait for his arrival. What purpose did it serve? What were humans supposed to be figuring out? Like right now everyone is waiting for all this stuff to happen for his reign.
Heb 10:12 describes Christ's sacrifice as being one sacrifice for all times, including the times prior. So, according to this logic when it took place isn't really the issue.
I have heard theological ideas (can't rattle off scriptures off the top of my head on this though) that explain how faith (apart from the Law) had always been the basis for justification & salvation. In other words, Christ had people covered through their faith in God even though they predated his sacrifice.
As for the law, etc., Paul taught that the whole God-dealing-with-Israel thing was a case study to show how mankind falls flat under his own effort, illustrating the need for Christ. Hebrews 9,10 goes on to show how the whole legal sacrifice requirement was just an illustration of the real deal, Christ's sacrifice, and couldn't really wipe out sin itself.