Given that God exists, it's reasonable to assume not only that God has revealed His moral will, but also the He has provided us with the means to know it. And, not only is it reasonable to assume, but it is in fact what God has done.
First of all, God has revealed His moral will in the conscience of all human beings (Rom. 2:15). Without going into much detail on the subject, the conscience is was not intended to be an infallible guide to morality. It's primary purpose is not to resolve difficult ethical question, but rather to provide testimony to secure the just condemnation of those who trust in their own merit in defiant self righteousness before a holy God (Rom. 1:32).
Secondly, the only practical guide to God's moral will has been revealed in the Holy Scriptures, the canon of which has been long established by the Christian Church. Having the sanction of divine authority, the Holy Scriptures are the only rule of obedience that God has provided us.
The epistemological basis of God's moral will is not empirical as, for example, we know the objects in our day-to-day experience. Nor is it rooted in those rational principles that God has provided as preconditions for making empirical knowledge possible. Rather, the epistemological basis of God's Word is faith: "Now faith is the substance of things hoped for, the evidence of things not seen." (Heb. 11:1)
Those who don't have faith may wonder in what way it may be considered a basis of knowledge, but that's not something that can be simply explained philosophically, because it is of divine origin (Eph. 2:8). It would be like trying to establish the epistemological basis for knowing that pain is an unpleasant sensation. No amount of argumentation or description can do the subject justice beyond the experience itself. Nonetheless, in no way should it be considered epistemologically inferior to the means by which we know any other type of knowledge. Being a matter of divine revelation, God has provide His people with a solid foundation for their spiritual and ethical knowledge:
"And I, brethren, when I came to you, came not with excellency of speech or of wisdom, declaring unto you the testimony of God. For I determined not to know any thing among you, save Jesus Christ, and him crucified. And I was with you in weakness, and in fear, and in much trembling. And my speech and my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power: That your faith should not stand in the wisdom of men, but in the power of God." (1 Cor. 2:1-5)
Finally, repentance is a necessary prerequisite to faith. It's unreasonable to suppose that those who unrepentantly reject God's revelation in the conscience should expect the means to understand the greater revelation of His Holy Word. Those who rejected John the Baptist's baptism of repentance also rejected Jesus' other teachings (Luke 7:28-30).
Morality could well be objective but ultimately we may never be able to know it fully. This view is best, because then we have the idea that there truly is a right and wrong involved and then try our best for it, but we also understand we could be wrong, in fact almost certainly will turn out to be wrong about certain things.
Moral relativism encourages people to just be like "well it's relative so I'll just stick with my beliefs since they can't be wrong." It encourages people not to take morality seriously at all, and become selfish. It's better to acknowledge that objective, real morality exists but that it is not perfectly knowable and never will be but that we may still do our best.
For thousands of years, philosophers have developed theories and doctrines that they believe defines morality. Most of the time, moral philosophy has attempted to discover universal principles, but universal morals is not 100% attainable. Everybody has different opinions on right and wrong, so morality is predominately a subjective matter, although there are a few ideals that can be defined as universal.
Morality is is in the eye of the beholder, or the individual. We tend to agree on certain points of morality as a society but one true morality does not exist. Morality is a personal opinion, even killing someone is subjective. Most any idea of right and wrong is subjective.