I actually disagree that morality is objective. While stealing, rape, murder, and cheating are considered fairly universal "bad things" by humans, I don't think it is because of a God. My theory is that these behaviors are dissuaded because the practice of dissuading "bad" behavior is an "evolutionarily stable strategy."
Meaning: Genes are responsible for code something in our brains that give us a sense of right and wrong. These things enforce behavior that makes us hold
other people to a higher moral code, and guilt them if they mess up. There are many, many examples of this.
Studies on anti-abortion activists who get abortions,
Self-serving bias, etc. The saying, "Do as I say, not as I do," is a result of this human tendency.
The reason we have these genes is because thousands of years ago, people who carry these genes were more fit. It is easy to see why - if I can convince people around me to not murder/cheat/lie/steal then it is likely that I won't be murdered/cheated/lied to/stolen from, and therefore can devote more resources to my children.
This can be proven to be evolutionarily stable since
even monkeys have a sense of right and wrong. Many other species also show reciprocity and shun cheaters.
It is important to note that the bad behaviors themselves still exist. This is because having some proportion of a willingness to do bad things can still improve the fitness of an individual. From a mathematical model, we can conclude that human beings are coded to be good most of the time and scumbags some of the time. And we all have different levels of goodness/scumbagness that can be affected by nature/nurture/situation.
In conclusion, I think that God serves as a gap-filler for holes in human knowledge. When there is a logical and mathematically sound explanation for why something occurs, then science should take over. For example, I disagree with
this article because while I agree there are holes that science can not yet answer, the key word is "yet." Remember that humans used to think the sun was a God. In a few hundred more years, our descendents may look back and laugh. "I can't believe humans used to think the universe was created by a God!"
Anyways, I've digressed quite a bit. Please feel free to PM me. I love the study and application of evolution in mundane matters.
TL;DR - morality is not objective because evolution.