Eternal life has always been by faith. That is what the scriptures teach. The old testament saints who "without us would not be made perfect"...all had faith. As apart from those who didn't have a living relationship with God by faith, but who nevertheless kept the Law. If we learn anything from the Old Testament at all it is that keeping the Laws of God resulted in blessings in THIS life, which are only reflective of the spiritual blessings to come with the New Covenant ( which include eternal life). That's why Isaiah cried out that though the number of the Israelites be as the sand of the sea, only a remnant would be saved. Saved because they had FAITH, not because of Law keeping apart from faith.
This might be a bit controversial, just putting it out there, but I have wondered about the possibility that what we are calling born again is on one level a conception...like Mary of who it was said, "that which is conceived in her is of the Holy Spirit". We have eternal life, but Christ in us needs to be born....to be made manifest, to be fully revealed. I believe that is what we are aiming for even if it doesn't manifest in this life. When Jesus comes He will be revealed IN His saints. I think there are kind of two levels of truth here, with being born again, if only I could explain it properly. I struggle to understand if this is possible in this life, or must wait for the return of Christ....whether there is the possibility of a personal level of Christ returning, as well as corporate historical. Or if that is just our hope....and the bible says who already has what he hopes for?
There is a reference in Hosea:
Hos 9:11
As for Ephraim, their glory shall fly away like a bird, from the birth, and from the womb, and from the conception.