People who make those claims presumably genuinely believe what they are saying, and the discussions they cause are often entertaining, and sometimes ludicrous, but, as with any discussion about the existence, or lack of existence of a god or gods, the end result of the debate is never, and probably can never be, conclusive. It all comes down to individual beliefs. I consider myself to be agnostic, anyway, so I don't support either the atheist or theist viewpoint. This Ramble is not concerned with whether or not religion is based on truth. I have wasted far too much time on such discussions already, and I now tend not to become too deeply involved if I can avoid it.
My question here is whether it's possible to lead a decent, moral life without the need for any form of religious guidance on how we should behave.
Throughout history it appears that groups of animals, including humans, have over time developed basic 'rules' about how members of those groups should behave. Most species establish a form of 'pecking order' within groups, so that the strongest member is first to eat from a prey animal that has been caught and killed, for example, and the weaker members have to wait their turn, and eat whatever is left. This may sound unfair, and it probably is, but it does serve a very useful purpose. It stops every hunting success turning into a mad feeding frenzy, where all the members of the group fight each other for what they can grab. From the overall point of view of the group as a whole that is counterproductive, because any members that are killed or wounded during the fighting reduce the strength and hunting abilities of the group, and therefore the group's chances of survival. A group following the pecking order principle is presumably more likely to survive and prosper than one which fights over every kill, so it's reasonably logical to assume that eventually that is the way that all groups of that species come to behave. A 'Rule' has developed simply by natural selection.
Herd animals in areas where there are large predators also display behaviour which helps the herd to thrive and prosper, even if it does so at the expense of individual members. Elephants, if threatened, will form a defensive circle, with the calves and young animals in the centre. I imagine this 'rule' of protecting the young, while possibly losing older members of the group, must also have evolved by natural selection, because a group which allowed its young to be killed would be less likely to survive than one which lost older members, which had probably already mated and produced young.
Those 'rules' are equally relevant to humans, and would probably have been even more important in ancient times, when the survival of the species was less certain, than they are now. In humans, the same kind of reasoning can be applied to many of the various 'rules' which society adheres to. Most human societies prosper by using peaceful co-operation between the members of those societies. In a group that depends on hunting, working together in an organised way is the most efficient way of catching prey, as even animals such as dingos have discovered. In a group that depends on farming, the labour is usually too much for individuals, so again co-operation between members is needed to produce the quantities of food needed by the group.
A person who disrupts that co-operation by killing other group members is a drain on the resources of the group, so the killer becomes a liability, and is punished or executed for the long term good of the group, and over time "No killing of group members" becomes another rule. Other actions, such as stealing grain or other food from communal stores would also reduce the resources of the group, threatening its survival, so "No stealing" would presumably become a rule. A male trying to mate with another man's female would also be likely to lead to conflict, with the possibilty, in earlier times, of one or other being killed, again weakening the group, so it's quite likely that a rule along the lines of "Thou shalt not covet thy neighbour's wife" might be created.
Over long ages, a large number of Rules are likely to develop within each group, all for purely practical reasons, with absolutely no need for any supreme intelligence to be involved in the creation of those Rules. I'm not saying there is no Supreme Intelligence, or God, or whatever, because in my view there is no definite way of knowing. There might well be such a being, or beings, who may have passed on their own set of Rules for Mankind to live by, but their existence is certainly not essential for the creation of rules for decent, moral and successful living. In other words, Atheists can be just as moral as Theists.