Usually this is not a topic I deal with, but an UD commenter gently asked such interesting question in another thread.
Theism states a transcendent Principle, which is One and Infinite. This Unity is the First cause of the universal existence, of all beings and all things (for this reason He is also the Great Designer of the universe).
This Supreme Principle is the Self “who stays within the heart of any being; who is the principle, the mean and the end of all beings”. Besides, this Self is also absolute Truth and supreme Knowledge.
Given this fundamental Unity, this Center, where “all beings are fused but not confused” – as M. Eckart said -, it is straightforward to derive moral principles for all men.
The first moral principle is “love”. In fact “hate” can exist only where there is multiplicity and antagonism. In Unity no multiplicity and no antagonism, then no hate. Moreover if a being practices “love” he faster approaches Unity, which is his final end. “Love” is the first moral principle in theory because conceptually it is directly derivable from Unity, God. But “love” is also the first moral principle in practice because it is what helps more the beings in their path towards God. This is the reason Christ preached love, Ramakrishna said “love is the easiest method of spiritual realization”, and all orthodox religions tried to establish love among men.
(Nota Bene: the fact that in history some religions were implied in wars and persecutions is not due to wrong principles, or principles of hate, rather to bad application by some of their representatives. Unavoidably all organizations are made of men, and also if the organizations are based on good principles, men – who are weak – may behave bad. But one cannot charge the organization’s principles of the misdoings of its members.)
All other moral principles are corollary of love and all are stated to help the beings to reach Unity. In facts to practice virtues leads to Unity, while to practice bad habits withdraws from Unity.
The strict connection between Unity and Knowledge can help to understand why so often ignorants are also evil. More a being knows (in the spiritual, intellectual sense) more is good. In fact he sees all other living beings as his own “brothers” in Unity, the shared Center of all things. “For whom stays in the Center, all is unified, because he sees all things in the unity of the Principle”.
In short, morality derives, almost mathematically, as an application of metaphysics because metaphysics is the “doctrine of Unity”, and all evils, all bad moral habits, pertain eminently to multiplicity, egoism, antagonism, disconnectedness and the instinct of disunity of individuals.