From Got Questions:
Morals are our definitions of right and wrong: the lines separating good behavior from evil behavior. Morals are not an explanation of how things necessarily are, but a description of how things ought to be. This implies a level of obligation. Labeling something “moral” means we ought to actively pursue it, while something “immoral” ought to be actively avoided. When we call something “moral,” we associate it with concepts such as “good,” “right,” “proper,” “honorable,” or “ethical.” The nature of morality also means that the arrangement of those moral lines—the way in which those concepts are arrayed—is itself a moral imperative, since that which is “not moral” is to be actively opposed.
Truth is our definition of reality: the lines separating what is real from what is not real. Truth is an explanation of how things really are, not how we wish they were or even how they ought to be. When we refer to “truth,” we evoke concepts such as “actual,” “real,” “factual,” “genuine,” or “existing.” The nature of truth means that which is untrue, or false, either does not exist or cannot happen. Truth is its own imperative: a person can either accept it or reject it, but it cannot be altered by opinions.
On the surface, morality and truth seem to occupy separate spheres. Truth describes what “is,” and morality describes what “ought to be.” Speaking of “moral truth” implies a combination of those two ideas. A moral truth would be right and good, as well as actual and real. Of course, since “what is” and “what ought” are not necessarily identical, the question arises whether “moral truth” can exist in a meaningful way, and what it would look like.
As it turns out, understanding morality requires a similar approach as any other set of facts: it is either objective or subjective. Objective morality—also labeled “absolute morality”—implies something fixed according to an unchanging perspective. Objective moral principles are linked to an unmoving, universal point of reference. Subjective morals—also called “relativism”—are linked to some changing, shifting, or preference-based perspective.
One problem with “subjective morality” is that it quickly becomes a contradiction in terms. If the lines defining what is right and wrong can be moved, then the purpose of morals itself is lost. One could conceivably call the same choice, in the same situation, either “moral” or “immoral” according to different points of reference. That in itself defeats the purpose of morality. Practical decisions might be entirely reversed, in that case. That subjective morality is self-contradictory implies actual morality is tied to something objective. That is, it is more rational to say that “moral truth” exists than to say that it does not.
Ultimately, the only reasonable basis for moral truth is God. An un-created, unchanging, perfect standard would fit the definitions of both truth and morality, simultaneously. Any basis for comparison or judgment eventually relies on an assumed “absolute” standard. Whether the concept is that which “is” or that which “ought to be,” the only reasonable basis is God. This means that which God calls “good” is the standard of morality: that is “moral truth.”