so i recently read a bit of Plato's Republic and i came upon a really good question.
this is a question that was proposed nearly 2500 years ago.
Are things moral because the Gods say they are, or are things objectively moral and the Gods just point them out?
the conclusion is if things are moral only because the Gods claim, then what if the Gods say something that most people think is immoral such as slavery (Exodus 21: 20-21) or child rape (Numbers 31 17-18).
if however the morals are objective and the Gods just inform us, then why the Gods? couldn't we just figure it out by ourselves?
The pagan concept of gods is very different from ours. To start with, they had lots of gods, who all had powers which were sovereign over mankind. The only way a person could escape the wrath of one god was by getting another one with more power to fight against that god. It is the world of the playground bully, and of gangs fighting one another. It is, in a sense, the world of the mafiosi.
However, just as with the mafia, the pagan gods are not omnipotent. They each are under the sovereignty of other forces; the fates, time, fortune. Depending on the whims of these other forces, things can either go the way the gods plan, or not.
In such a context, morality is a difficult thing for Plato to consider, because the pagan gods are NOT particularly moral, but instead are very human in their desires and their behaviours. They fall in love with people, and they come from Olympus and attempt to seduce. If that fails, they rape instead. If both fail, then they punish their love object without mercy. These are not the kind of gods to define morality for us.
Therefore, to Plato, Morality must exist over and above the gods, and must be capable of being defined separately from them. The only alternative is to accept a bendy kind of morality, which sways with the wind, or more accurately with the animal passions the gods display.
So then we come to the Judeo Christian God. This is not a God who is ruled by his passions as we are. He is a God of love, but that love is not the love which Zeus demonstrates. In Christian terms, God is the source of all morality, because he is incapable of acting immorally. Therefore, we are safe in using him as the benchmark for our behaviour, and saying that morality is invested in him, and is found only in him. When we attempt to behave in accordance with moral principles, we are seeking to emulate the Deity, or alternatively, we are allowing the Divine within us his due sovereignty.
Human ideas of morality change, as you rightly say. What one generation accepts, the next will reject out of hand. What we ought to realise is that as we change our morality we do not leave God behind, in some kind of state of impotent wrath towards our sinfulness. Rather we move closer to his actuality; the actual love, mercy and acceptance that Christ demonstrated, and that our society inches towards, bit by painful bit.