By Barry Goldberg, Born Jewish, Raised Mormon, Discovered Philosophy and Became Atheist

Theists often like to use the idea of objective morality to prove the existence of the particular god they happen to worship by way of a bit of circular reasoning. First, they assert that objective morality exists. Then, they claim that objective morality can only exist if provided by a source external to humanity (a source which they further claim can of course only be the particular god they happen to worship). They then conclude that the particular god they happen to worship must therefore exist. Q.E.D.

Except, when pressed to justify the claim that objective morality exists in the first place, they can only point to their beliefs that the particular god they happen to worship exists and has provided those objective morals to us. In other words, “I know that objective morals exist because God provided them to us, and I know that God exists because we have objective morals.”

The reality, however, is that we don’t actually know that any sort of objective morals actually exist. To be truly objective, morals would need to apply equally to all cultures and all time periods and not change from place to place or from time to time. And we simply do not observe this in the world. Now, don’t get me wrong — an argument could certainly be made that humans have a natural inborn sense of empathy that could conceivably provide the basis of some sort of morality common to all cultures, but that’s not certain. What is certain, however, is that we absolutely cannot look to religion to provide us with any sort of objective morality:

• Different theists believe in different Gods, each of which is said to have given different moral laws for us to follow. So, right there, theistic morality is wholly subjective based on which God you believe in.
• Even within a single God belief (Christianity, say), there are tons and tons of different denominations and sects who all interpret the supposed “word of God” in different ways from a purely doctrinal standpoint. So, once again, even within the Christian faith, theistic morality is wholly subjective based on which particular sect or denomination you belong to.
• Even within a single sect or denomination, it’s pretty much guaranteed that different preachers or even individual members will have their own specific interpretations as to just what their God wants them to do. Should you shun homosexuals or welcome them? Should you donate money to homeless people or is that just encouraging bad habits? Do women really need to be subject to their husbands’ will or not? Is it enough to just accept Jesus into your heart, or do you actually need to do good deeds and repent for your sins? Is it really harder for a rich man to enter the kingdom of heaven than it is for a camel to pass through the eye of a needle, or is that just a metaphor? Does “turn the other cheek” mean you can’t own a gun for self-defense? Did God really just promise to “answer prayers” (and sometimes the answer is “no”) or did he actually promise to give “whatsoever we ask for in faith”? Is lusting after a woman in your heart really the same as committing adultery, or was Jesus just being metaphorical again? What’s the best way to “love thy neighbor as thyself” while still preventing transgender people from using the bathroom they feel most comfortable in? Is it OK to vote for somebody who claims to share your values if he talks about sexually assaulting women, mocks disabled people and lies all the time? What, actually, would Jesus do? And so on and so forth. Thus, theistic morality is wholly subjective based on the individual beliefs of each particular theist.
• For theists that claim to get their morality from holy scriptures written thousands of years ago, many of the oldest commandments and moral codes from those books no longer apply today. The explanation for this is usually that those commandments were given for a specific group of people, that the culture and socio-economic conditions back then were different than they are today and/or that some sort of “new covenant” made those old commandments obsolete. It was OK to own slaves back then, but not today. It was commanded that disobedient children should be stoned to death back then, but we don’t need to follow that commandment today. Jews were required to keep kosher, but later Christians didn’t need to. All of which is to say that theistic morality can actually change over time and is wholly subjective based on the particular people to whom the moral commandments were given.

https://essaysonatheism.quora.com/Objective-vs-Subjective-Morality