To paraphrase Maritain: "...because all law is a promulgation of reason, ... there must be an eternal reason behind the eternal law; and since the natural law is nothing more than our participation in the eternal law, there must be some principle of reason behind the natural law."
Morality is value-laden action. Action is behavior ordered towards an end or ends.
If moral claims are absolute, they are also eternal. That is, if there are absolute moral principles, there are absolute principles of action.
There are absolute moral truths, and therefore also absolute principles of action.
Action, however, cannot exist without an agent. As such, absolute principles of action exist only because an absolute principle of agency exists. Morality is absolute, therefore there exists an absolutely moral agent, which all men call God.