The title reveals my assumption that these souls experiences a pre-mortal life, which extended back far beyond the existence of Man. Therefore, all Men, whenever and wherever they were born and lived, will have known about the 'plan of salvation' involving Jesus; and when each soul incarnates as a mortal Man, will have known the range of possibilities for that time and place.
Until Jesus became divine (when he was baptised by John) Men's souls could not become divine - they could not be resurrected to eternal and Heavenly life. From this point all souls of Men could become divine, by believing/ having-faith-in/ following Jesus - and all souls knew-about Jesus from their experiences in pre-mortal life.
Before the divinity of Jesus, souls were 'waiting' - at least, that seems to be the essence of pagan beliefs. It may be that some dead-souls (after mortal death) simply awaited in an unconscious and dreamlike state; probably some souls reincarnated to gain further mortal experience (in that probably the majority of pre-Christian 'religions' assume some kind of reincarnation).
My general conviction is that the plan of salvation is one that allows for unique specific trajectories, tailored to the unique dispositions and potentials of each human soul, but there are a few stages in the evolutionary development which must unfold linearly.
It seems likely that some experience reincarnation (of various types) while others do not. Some choose eternal resurrected life after one mortal incarnation, and for many this incarnation is extremely brief and takes place entirely in the womb.
And within divinity, there are many levels between ordinary Men and the fullness attained by Jesus, and some will probably choose to remain at lower levels.
And some Men choose damnation - quite likely these souls were already very-probably corrupt, and mortal life was a kind of 'last chance' for them. I suspect there are a large number of such premortally evil souls incarnated at present - at least, that's how it appears to me. Modernity is, overall, perhaps designed to make the issue of salvation versus damnation as clear as possible to such souls (the phenomenon CS Lewis called 'things coming to a point').
As for the souls born before Christ; and indeed until modernity - I don't think we need 'worry' too much about their hopes of salvation. Each will have known Jesus in pre-mortal life (as well all did) and almost all will have lived in a society sufficiently 'Good' that they can make the right choices - there was a great core of commonality in perspective, including morals, (what CS Lewis called The Tao) between all known traditional religions.
The kind of mainstream, official, inversion of Truth, Beauty and Virtue which we Westerners (and increasingly everybody else) experience as our everyday life, was extremely rare during past times; but has become the prime shared experience and the main challenge of our mortal lives.