there are philosophers that have tried to find a supply of goal morality that does not come from God. The one I'm a bit acquainted with is Kant and the express imperative.
Kant argues that the only objectively proper thing is a accurate will. The consequences of your motion don't be counted to verify the goodness or wrongness of your action, because ends depend on particular occasions and inclinations. What is essential is that you acted out of duty. This obligation comes from Reason alone. In a nutshell, you need to solely act in a way in such a way that you can at the equal time desire for the maxim of your action to end up a regularly occurring regulation without contradiction (for example, don't lie, because you cannot wish lying to become a regular law, as it would be a contradiction to the idea of communication. Eventually no person would accept as true with what the others said).
Now, of course the query that without delay comes is "and why need to I [act in such a way that...]?". Here, it is well worth noting that even in the case that God existed I could ask the equal question: "why I accept his morality as absolute?" Even if a God existed, why would it be objetively desirable to comply with his precepts? The answer would be: because he is God, of course. By definition, he is the supply of morality, so not obeying him would be a contradiction that would go towards the essence of a being created by means of God. Just as in the case of the express imperative, not obeying it would be going in opposition to the essence of what consitutes a rational being that excercises practical reason.
Notice that in each cases of claimed objective morality the answer to the question "why ought to I comply with these rules" is "because in any other case it does not make sense". So, my conclusion is: even if God exists, Reason is nonetheless what tells you that some thing is top or wrong.