A common argument that one frequently encounters in religious debates is that: without religion, there can be no objective morality. You've probably heard William Lane Craig make this argument before.
The first thing to point out is this: even if we granted for the sake of argument that this were true, this wouldn't do anything to validate a specific religion or demonstrate the existence of god. The Bible may tell us that "thou shalt not kill"; but the fact that the Bible contains this moral guideline tells us nothing about whether any of the stories contained within that book are accurate.
Not even Ray Comfort would say something as ridiculous as: The Bible says "thou shalt not steal; therefore, Adam and Eve literally lived within the Garden of Eden 6,000 years ago." The conclusion simply does not follow from the premise. So when you ask a religious person to provide you with evidence of their gods existence, and they say "objective morality comes from religion", they've just made a non sequitur argument and committed a logical fallacy.
There are more problems with this argument. If we take the Bible as an example, it's clear that the holy books themselves don't, in fact, provide a concrete, unquestionable set of rules. There is so much room for interpretation in the Bible that there are hundreds of different sects of Christianity. Take any moral question—abortion, stem-cell research, contraception, gay marriage—and you will find that there are denominations and members of Christianity that hold contradictory viewpoints on each and every one of these questions. How could a religious text that's so incredibly broad and vague, which allows for such a wide spectrum of beliefs, be called anything close to "objective"? This is about as unobjective as is gets.
And keep in mind we're only talking about one religion here. The problem gets amplified when we consider that there are hundreds of religions, most of which have contradictory beliefs and which differ on their answers to certain ethical questions.
So when you actually examine the moral beliefs that religious people hold, it's clear that religion doesn't provide us with a set of objective morals. And even if it did, that wouldn't do anything to demonstrate the existence of the god that they think provided us with those morals.