What about those too young to understand Jesus?
What about all those born before Jesus?
What about people born in the ME, India or China today, Buddhists, Hindus, Jews and Muslims, that have never learned or even heard about Jesus?
Excellent question.
What Jesus said at the time, "If I had not come and spoken to them, they would not be guilty of sin." which implies that those who never heard of him, were too young to understand, or were born and died before he came were also not guilty of sin.
By the time Jesus appeared the truth had been lost to time for a thousand years...
To be guilty of the sin of unbelief, one would have to have heard his testimony which implies understanding, and then rejected it.
By that standard no one alive today has ever heard or understood his teaching.
That too had been lost to time a thousand years before anyone alive today was born.
He would have to reappear and clear things up before anyone would be guilty of the sin of rejecting his testimony.
Refusing to believe in what cannot be rationally explained or understood is not a sin. Thats actually the right way to lead a kosher life.