Hello Isaiah. Good to hear from you. There are many good books on this topic such as Mere Christianity where C.S. Lewis includes some thoughts on this subject in his book, and The Abolition of Man, where he deals directly with what he calls “the poison of subjectivism”. Ravi has written on this in several of his books and there are messages online which may be of help.
Historically, one of the main views was that truth and morality are objective things and conformity to them was considered central to good living. There needs to be a standard by which we judge things and situations, some criteria that is the measurement we appeal to, in order to call anything good or bad. If all is subjective and merely a question of taste, preference, situations or diet??? Then we have no means to say one thing is better than another. It is just different. Some people in some countries are taught to love their neighbors, in a few places they were encouraged to eat them! Is there really no objective difference? Are our choices totally dependent on situations?
Some situations do make a difference? Here we need to consider a hierarchy of goods. What is the lesser evil facing the person dealing with the Nazi? Now some would assert that in all cases under all circumstances you must tell the truth. Of course there are consequences. There are consequences anyway, to all views. Others would assert, that the lie in this case, used selectively and given the dire nature of the issue, protects a life, so a higher good covers the lower issue. Again, some would take issue with this, which is why moral reflection takes a lot of serious work.
The illustration of the wife and her choice of adultery is more like an act of revenge than a moral outcome? Is counseling an option? Are there other possible, better, less extreme options than creating a moral equivalence between two bad behaviors?
Our innate moral conscience seems to point to the existence of real, external, objective truths. Even those who deny them often end up invoking them as the standard by which they judge (in their case objectively) your wrong definition of ethics. Many years ago, the philosopher Immanuel Kant said that two things convinced him of the existence of God and bore witness to His reality. The starry heavens above and the moral law within.
There are many resources on the RZIM website that can provide a more in depth and more robust response. I hope for now, this is a little help.