If God were the source of morality, then morality is not objective but instead mind dependent, i.e. based on God's subjectivity.
If the God cannot change his morals on a whim, God is not all powerful.
If the God of the bible is the source of morality, then it is moral to rape, pillage, genocide and also to murder babies.
Seems morality is a human construct based on the observable notion of cause and effect - and since we're a sentient, communal and empathetic creature, morals are based on the mitigation of suffering. This is proven over and over by the fact that over time, our morality has evolved and things that were considered moral hundreds of years into the past, or even just decades, are now considered barbaric.
Also, as secular society evolves in this way, theists have been conforming to the new secular standards, and have been forced to reinterpret much of the old-guard morality of their ancient scriptures.