What is objective morality?
The beginning position is to define "objective". Objective in relation to morality in simple terms means a set standard against which actions can be judged. That set of standards have to first be chosen. "Choosing" is a very important element of objectivity.
My position is that objective morality is whatever a society or group of people have chosen it to be, and that choice is wired into offsrpings of those societies or groups as the standard against which morality is judged. For the Christians they have chosen the Bible as the standard against which morality is judged.
But the Christian's choice is a religious book. It is a book written by men but which they choose to believe is from the deity they worship. So a Christian's source of objective morality is what they choose it to be.
Is the Christian's choice perfect? No.
Their chosen source of objective morality does not condemn slavery. It subjugates women, calls them unclean for menstruating. It says divorced women must remain single and cannot remarry. It says to obey monarchs. If revolutionists were 100% subservient to that source of objective morality, they would not have rebelled against the monarchies and autocrats cos their source of objective morality asks us to obey monarchs and autocrats cos they are God's representatives.
Morality is a social construct guided (not created) by the religious beliefs the group or society decides to follow.
One good illustration is cousin marriage. In my society, it is immoral and an abomination to marry your cousin. Cos I grew up in that society my brain is wired to be repulsed at the idea of sexual relationship with my cousin. But there are societies where it is moral to do so.
In sum, "objective" is whatever we've chosen it to be. And no choice is perfect. Those set of standards against which morality is judged will continue to change as human societies change, and sometimes those changes are in form of reinterpretation of religious texts.