So before jesus, no one reached heaven? It was just empty?
That question calls for a LONG, complicated answer. I'll try to do the short version. Here's an interesting quote that attempts to explain it from one angle:
"Paul mentions in Romans 3 that Old Testament believers were forgiven "in the forbearance of God" (Rom 3:25). A forbearance is a postponement on a debt—like when a student graduates from college but doesn't have a good enough job to repay his student loans, the feds may say "We'll give you five years— but then you'll have to start paying." In the Old Testament, God gave his people a forbearance until Christ could come and pay their sin-debt for them. In this way they could avoid the punishment for their sins, even though Christ had not yet died for them.
When Paul introduces his doctrine of justification by faith alone in this same chapter, be backs it up with the Old Testament, saying it's a "righteousness from God apart from law," but one "to which the Law and the Prophets testify" (Rom 3:21). The "Law and the Prophets" is a technical term for the Hebrew Scriptures, our Old Testament Paul specifically backs up his teaching by citing Abraham's justification by faith from Genesis (Rom 4) and David's speaking of the forgiveness of sins in Psalm 32 (Rom 4:7-8).
I think that is actually a pretty decent summary without getting too deep. Essentially, it was God's plan all along that Jesus would pay for the sins of all mankind. So for those who lived according to faith in Him
prior to Jesus, God forstalled judgment until Christ died and actually paid the price that allowed them to be saved.
Put another way, God forordained it before creation, so it was as good as a done deal. This, I believe is why in revelation, it describes Jesus as "the Lamb that was
slain from the creation of the world." (Rev 13:8)