Those who believe in God advance the 'highest' moral system created: the one by their respective God. Even when thinking they are granting a concession to nonbelievers regarding the ontological nature of God, they still press that even if God didn't exist, there would be no objective moral system, leaving only moral relativism. The Good is decided by God, and without God, there is no Good. However, what they fail to realize is that by holding up a God who decides 'The Good', they are actually enshrining the ultimate moral relativism. Their moral relativism would be actually worse for it would be systematized, making a 'tyranny for our own good' while individual moral relativism would be constrained to the individual.
An objective moral principle of humanity and individualism states that no one is the property of another. However, God and His 'prophets' had slaves of all types. Muhammad had numerous slaves, and the Bible makes references to the owning of slaves (including sex slaves - remember Moses' taking of the 32,000 'women who had not known a man) to which neither Jesus or Paul explicitly condemn. Objectively slavery is immoral, but with God and God's will, slavery is permissible.
An objective moral principle of humanity and individualism states that murder is immoral (murder, not killing for murder brings with it its own context as killing is vague). However, God and His 'prophets' murdered countless people. 'Killing in the name of...' is the [appropriate] pejorative, but also the justified excuse believers use. Whether it is killing the apostates and nonbelievers mentioned in the Koran, the 'sinners' in Sodom and Gomorrah, the children of the subjects of the one who isn't liked (parents of firstborn in Egypt who wouldn't have any significant political power), the entire planet in Noah's story, or an individual that is one's own child with Abraham - murder is acceptable or even held as an exemplar of devotion with God. God had 32 of the 32,000 virgins offered up to Him. Objectively murder is immoral.
The Classic Greeks asked this question with Plato and his Euthyphro: is The Good what God (the gods) decides, or does God (the gods) like The Good because it is The Good. To give the base of morality to any [Abrahamic-based] God is to cede morality to an interpreted understanding of what others advance from books written by man, influenced by the culture/context of the time, interpreted through generations today to come to their understanding to be pressed upon today. It is relativism, plain and simple. Objective morality based upon human rights and individualism is based upon principles which are eternal, while understanding of 'God's' will fluctuates with time and to who is being addressed. If you want an ultimate, objective standard of The Good and morality then it is not to any God that one should be turning to.