They are going to roll their eyes regardless of what you say. Just tell them the truth when necessary and disregard their mockery besides making reparation for it.
I would agree with that.
If you can’t convert someone to your belief by logic, you may be able to help them understand why you believe what you do, and that your beliefs are not inherently unreasonable. This can give them food for reflection down the road.
At best, I would begin by asking if they believe in the existence of truly evil behavior in the world. They may have trouble denying that, as it seems commonsensical to anyone who watches the news, and it is an integral part of their chief argument, that the presence of evil and suffering in the world makes it unlikely that there is a God who would permit it.
If they hold to a truly relativistic view of morality, that all behavior is cultural or “evolved” and thus there is no true evil, they would deny that Nazis, ISIS, child torturers, etc. were truly evil. Most will not be willing to go so far, at least in front of others. (Some are, and you may find you have no common ground for discussion at this point in their lives.)
If they accept that, you could ask if they believe in the objective existence of moral absolutes - i.e., do “Good” and “Evil” have some sort of existence that would be present even if there was no sentient being in the universe?
Most atheists will say no, as they are materialists and will be uncomfortable with the idea of moral absolutes existing as some kind of platonic ideal.
Suggest that moral qualities, such as Good and Evil exist only insofar that they are embodied as the attributes of a being that is capable of making moral decisions. When a lion kills a gazelle for food, or a cat kills a bird for sport, or a rock rolls down a hill and crushes a man, we do not consider that “evil” because we don’t consider animals or rocks capable of evil moral action.
We as Catholics consider all created things as having an origin (God as the sole uncreated being is the necessary and sole exception to that, which will likely by a side argument - don’t get distracted by this but be prepared to defend it), and thus, all good things originate from God, who is himself Goodness - the sole standard, or measuring stick if you will, for good actions. (This is also part of the best rebuke to a common atheist claim, that the Euthyphro argument is a true dilemma) Among those goods is the immeasurable gift of free will, which allows His creations to choose between Good and its shadow, Evil.
You can finally suggest that we consider evil, and evil actions, to be embodied in a person, or being, as well, whom we call by several names (and whom numerous other religions recognize and acknowledge in their own way) but who seems to us to be the source of much of the evil in the world. He is obviously less powerful than God and is not capable of creation, which is solely God’s provenance, but through his rebellious example, having been given the same choice to choose between Good and Evil (and through the other beings who give allegiance to him, and who made the same choice), and hates and seeks the ruination of mankind. He can be said to be the source, through his actions, of the evils - pride, anger, hatred, etc., that give rise to other evils. He does not directly tempt or cause every wrong that happens in the world, but most everything can ultimately be traced back to something else, including Evil.
They are unlikely to accept this if they hold to a solely materialist view of the world, but it will give them something to think about and perhaps help them to understand why our belief is not unreasonable, given our philosophical starting points.
