It has been claimed by theists that without god it is impossible to have objectively-based morality. Leave aside that doing what someone else says you should do is not an objective morality, how hard is it to make a basis for morality that is objective? Well I thought I'd try and it took it less than 10 minutes.
Imagine a world without rules, no morality, no law, no binding customs (although they might have habits). Obviously you would be better off with some system of rules to limit undesirable behaviour. One of my friends comes up to me and says "I want to be able to trade without fear of being murdered and my cargo stolen. What can I do?". I say well let's form a club with only 1 rule, if you murder someone in the club you are expelled. The only bad thing about being expelled is that members of the club can then murder you without consequence just as they can murder people who never belonged to the club. This club would be very popular. So would a club that had as it's condition that you don't steal from the other members. It is objectively true that if any of these clubs were opened in such a rule-free world I'd join them. I know this objectively because I have sufficent knowledge of my own preferences. These preferences are subjective, but my knowledge of them is objective. So if I base my morality on not doing anything that would get me thrown out of a "rule club" that I join it's objective morality.
Like I said, less than ten minutes.