
When people of different religions and of none have such different ideas of what is right and wrong, how is God going to judge them? What standard can he use so his judgment is fair?
This is a very good question. As you say, in today’s world there are big differences in how people see the morality of acts, including such basic ones as abortion, euthanasia, homosexual acts, etc.
Looking at it from another point of view, there are those who say that the question of what is right and wrong is completely subjective. That this is relative to how you see it personally in your particular circumstances.
It is often expressed in terms such as, “Morality is what you make of it” and, “To each, his own.” This is what is known as moral relativism.
On the other hand, there are others who say no, that morality is objective, the same for everyone. That independently of how you see it, certain acts are in themselves simply wrong and not to be done, and others are right and good.
How is God going to judge all these people? Will it be by what they think is right and wrong and how they have lived in accordance with their beliefs, or by some objective standard?
When we think about it, we would probably hope and expect that God would judge everyone by the same objective standard.
It just doesn’t seem right that, if we have made an effort all our life to refrain from doing what we knew was wrong, like being dishonest or greedy, we would receive the same judgment and reward as someone who thought it was okay to be dishonest and greedy and lived accordingly. This, we think, would simply not be fair.
Or, looking at it in another way, if someone thinks it is quite okay to steal my outdoor furniture or my car, to kill my handicapped daughter or to sleep with my wife, I would not say he is entitled to his opinion—to each his own. No, after all, some actions are just plain wrong, no matter what people may think. They are wrong always and everywhere.

Ancient philosophers like Aristotle, who lived in the fourth century before Christ, Cicero, who followed him three centuries later, and many others found an objective standard of morality in human nature.
The word nature, by the way, refers to what makes something to be what it is and not something else. So trees have tree nature, horses have horse nature and humans have human nature.
When we reflect on human nature, as the philosophers have done, we see that acts like stealing, fraud, killing an innocent person, etc., are contrary to the good of the individual and of society, and so they are simply wrong.
This is what has come to be called the natural law, because it is based on human nature. So much is this recognised universally that every society has laws which punish acts like these. Since everyone is deemed to be aware of these basic principles of the natural law, God can judge everyone by this same objective standard.
Although everyone is deemed to know the fundamental principles of the natural law, there are other principles of which some people may be ignorant. They were simply never told that acts like the use of contraception to prevent pregnancy, sexual acts by oneself or with another outside of marriage, etc., are wrong.
Since, as we all know, in order to commit a serious sin, one of the conditions is that we must have full knowledge that the act is seriously sinful, if someone is ignorant of this through no fault of their own, they are not held accountable for a serious sin before God.
Therefore, in the particular judgment too, God will not hold them responsible for those acts. God is just, and merciful, and he will not punish someone for something they did not know was wrong.
But since sin always hurts the sinner, and often others as well, we should make every effort to learn the principles of God’s law, so that we can live well and not be found wanting in the judgment.
The Catechism of the Catholic Church has a very complete treatment of moral life, and my book The Final Exam (Connor Court 2023) is a simplified commentary on it, based wholly on the natural law.