So, basically Jesus tightened up the entrance rules?
The rules never changed, it's just that more knowledge was revealed.
Faith in God has always been the condition. If you trust in God, you trust in His word. And until the time of Jesus, that had been the Old Testament or the direct word from God before then.
But even from the time of Adam, there was an expectancy of a messiah coming. The 3 fold prophecy given in Genesis addressed the fates of Adam and Eve (humanity), the universe, and Satan. Satan's fate would be that the seed (singular) of Eve would crush the head of the serpent (Satan). In other words, a descendant of Eve would be the one to deal the death blow to Satan.
Further on in the OT, most notably in Isaiah (which contains a whopping amount of messianic prophecies fulfilled in Christ) but spread throughout scripture, further details concerning the coming messiah and his role and attributes are given.
The Old Testament saints (believers) knew that a messiah would come, and they believed in Him. Even though they never saw His arrival, it was counted as righteousness or salvation from God. Just as we who come after Christ and have never seen Him yet still believe are accounted righteousness.
The issue I see you getting at is the sovereignty of God. If you believe God is the creator of the universe, then really He has every right to appoint some to salvation and condemn the rest to hell. When it comes right down to it, you are questioning whether a creator has full right to what happens to his creations. Or to use a biblical metaphor, whether a potter has right to create vessels for honor and vessels for destruction both using the same clay.
Really, everyone innately understand the sovereignty of a infinitely powerful creator over his creations, so it becomes far easier to deny the existence of a creator to begin with and live according to each person's own standard of righteousness (which leads inevitably to sin and destruction).