Many theists claim that there can be no morality without god. This is an interesting claim as the exact opposite is true: there can be no morality with god.
The argument is simple. If you have a gun and are faced with someone you loathe and despise and if you decide to not kill them because “God” (or the LAPD, or …) would punish you in the hereafter (or the here and now), is this a moral act? The answer is “no” as it is merely an act of self-interest. If you refrain from killing your enemy because that would be an immoral act by what ever moral code you operate under, then that would be a moral act.
To quote the book All God Worshipers are Mad: “Actions are moral only when they are performed because of their morality.”
So, the consequence of having an all-powerful god is that the all-powerful god trumps any human system of morality (e.g. it is moral because God commands it) consequently a god-based morality is one in which people can only act in fear of god. In other words, morality is only possible in the absence of such a god.
So, if someone asks you “how can their be morality without god?” Ask them how god enforces His Morality? You will then know that morality is impossible under such a system as no one would choose a moral system for any other reason than to avoid punishment or to extract a reward for the all-powerful deity. Don’t expect the “God Worshipper” to be convinced, though, they swallowed their Kool-Aid a long time ago and it will be hard to cough up.