Salvation Through the Ages, Chap. 3, The Age of Conscience
From the Fall to the Flood
Human beings had now acquired a conscience, an inner moral code showing the difference between good and evil. With the very first children we see the divide between them, and that individuals were held to account by God for their actions. We have only a mention of sacrifices, and by the fact that Cain and Abel were treated differently due to the nature of those sacrifices, we can deduce that it involved the death of an innocent animal— an unmistakable foreshadowing of the eventual sacrifice Jesus would willingly make.1
In the generations following, there was as yet no formal government. So each individual did whatever they chose, though we are given the cryptic clue "In those days people began to call on the name of the Lord."2 But as time progressed, people regressed— to the point that the Bible says "But the Lord saw that the wickedness of mankind had become great on the earth. Every inclination of the thoughts of their minds was only evil all the time."3 This brought on the Flood, which destroyed all but eight people, the only ones on the planet who found favor with God.4
Between the beginning and end of this age, something else was happening. As explained in Gen. 6, the "sons of God" mated with human women to produce very remarkable offspring. They are described as "Nephilim" who were "renown, the heroes of old". Surely there is more here than mere tales of adventurous humans. If we remember that scripture was not written in a vacuum, and that scripture is marking this as what led to a completely evil and corrupt world, we can deduce that these were hybrids, the mixture of fallen angels and humans. Such beings would be a very plausible source for the many myths of gods. There is also the possibility that genetic tampering went on between these hybrids and animals, giving rise to the stories of beings such as the minotaur.
Salvation as promised at the Fall in the Garden of Eden was for humans only. We have nothing in scripture to go on to say it was offered for any other types of sentient beings, good or bad. The Hebrew terminology describing Noah is more literally rendered as "perfect in all his generations", which takes on new meaning in light of the hybridization of the human genome. It may well be that only Noah was purely human by then, which in turn explains why God would wipe out billions of others. Perhaps Satan's intent was to prevent the promised Seed from coming.
So salvation in this age involved calling out to God in some way and bringing sacrifices. I think it is safe to assume that they knew more about this that was recorded in the scriptures. People still knew of the existence of God and did not doubt it at all, but of course this doesn't mean they all obeyed God. If Adam and Cain could talk directly to God yet take adversarial positions against him, the mere acknowledgement of God is no guarantee of salvation or righteousness. It was a matter of choosing sides, between God and Satan, good and evil. Even after much time had passed and knowledge of God grew dimmer, some people such as Enoch were pure and righteous in God's eyes. In his case in particular, we are told expressly that he was taken up to God and thus "saved".