The way I see it is, Jesus ( God ) came down to die on the Cross at the time He knew would be the most fruitful.
If His death could have saved more people three centuries earlier, He would have come three centuries earlier. But He didn't. He came when the time was ripe.
There are lots of people, even today, who APPEAR to be genuinely seeking the truth, and never finding it. Think of Kierkegaard -- a genius, with a brain like a Ferrari, and he never found the Catholic Church. He was probably my favorite writer when I was "searching." Just an incredibly stylish and intriguing writer. So who is to say Socrates, for all his great genius, would have accepted Christ? I'd say Kierkegaard came far, far closer to the truth ( he was a Protestant ); but he never got there, from what we can see humanly, anyway.
I won't venture any further than that. The ideas of invincible ignorance and implicit faith for the pagans BEFORE Christ is a little too touchy for me, a little too much of a terra incognita. What I'm trying to say is that human genius does not necessarily presuppose an open heart that is willing to accept God; in fact, it can stand in the way of accepting God, because it can lead to a worship of self, even one that masks itself as humility.