The last superstition of any importance is the belief that a god either provides or demands morality among humans. The idea of god-given morality is the last bastion of religiosity remaining after all other superstitions have been stripped away by evidence and reason.
Broadly speaking, a superstition is an incorrect explanation for a natural phenomenon. Very often this entails references to supernaturalism of some kind. When a cluster of superstitions is organized around a belief in supernatural beings, especially beings that we are supposed to propitiate, then it is a religion.
From earliest times, humans have imagined explanations for natural phenomena, concerning themselves mostly with phenomena that directly impinged on their lives and livelihood, but also more broadly, to anything they experienced. They assumed volition or agency behind these phenomena, and postulated the forms of the agents as supernatural beings. Thus, there was a god that produced thunder and lightening, there were sun and moon gods, and so on. Any natural phenomenon was susceptible to such an explanation. In addition, they explained many of their own behaviors in a similar fashion, especially behaviors that were directed toward the assumed supernatural beings. In this way morality, which is based upon a natural phenomenon, was assigned a supernatural cause.
Our basic moral sense is indeed a natural phenomenon. Specifically, our moral sense is an evolved sense that is innate in all humans. We have come to this conclusion from various directions. First, all human groups share basic moral tenets, such as the Golden Rule, the incest taboo, injunctions against murder, theft, lying, and dozens more. At the same time, of course, all cultures have developed other behaviors that they deem to be moral, but that we can reject as innate for several reasons: these cultural elaborations are as disparate as the cultures themselves, they are often contradictory across cultures or even within a culture, and they often represent beliefs that have been shown to be wrong or entirely meaningless.
Second, a basic moral sense is exhibited by pre-verbal babies, who have been shown to have a sense of fairness and a sense of justice. This is expressed well before babies have acquired cultural behavior.
Third, our closest relatives, the chimpanzees, also exhibit a basic moral sense, one of the multitude of features that we share with them. Studies of various social monkeys and even of dogs also have revealed the presence of a basic moral sense.
From these facts it is evident that morality, in its essence, is an innate, therefore evolved, therefore genetic, trait of humanity. Morality apparently has helped promote the survival of social individuals or groups.
Morality is a natural phenomenon, as real as any other. It can be sensed emotionally, and it guides our behavior, but it cannot be touched, tasted, smelt, seen, or heard. It is just there, and it is ubiquitous. It is transcendent (as religionists claim!); it transcends all individuals and all groups of any kind. There can be no wonder that early humans would be ignorant of its source, and no wonder that it was universally assumed to be the product of an invisible agent, a supernatural being.
We now can apply reason to the evidence. We know that morality is an innate property of humanity. We have an explanation; we do not need supernaturalism to explain it. The last superstition has fallen before rationality. We no longer need to believe in a god, and the last bastion of religious belief has become meaningless.