Consider the following argument.
(1) If God does not exist, then there is no divine lawgiver.
(2) If there is no divine lawgiver, then there are no moral laws.
(3) If there are no moral laws, then there are no moral obligations.
(4) Therefore, if God does not exist, then there are no moral obligations.
(5) Laws must be made by a lawgiver.
(6) A lawgiver must be either natural or divine.
(7) Moral laws cannot have a natural lawgiver.
(2) Therefore, if there is no divine lawgiver, then there are no moral laws.
But why should anyone believe (5)? Laws require a lawgiver only if they are, in fact, made. Statutory (governmental) laws are the paradigm example of laws that require a lawgiver, but, to use one of William Lane Craig’s trademark expressions, statutory laws began to exist. Not all laws are made, however. The laws of nature, logic, and mathematics are three examples of laws that are discovered, not invented. Not only do these examples undercut the support for premise (5), they actually provide the basis of an argument against (5), based on the following negative analogy.
(8). The laws of nature, logic, mathematics, and (objective) morality did not begin to exist.
(9) The laws of nature, logic, and mathematics also do not have lawgivers.
(10) Therefore, the laws of (objective) morality do not have a lawgiver.
(10) entails, accordingly, that premise (5) is false.
There are many additional points I could make here, but they aren’t needed. Laws do not need to be made by a lawgiver.