A comment brings up this overall question of what is good anyway? I suggested that humanity is good for the most part, but what is that actually measured by when an unbeliever says it? Well, obviously since I say it, then goodness is defined and measured by my own judgements. This obviously may not match at all with your own view. For example, if you believe a good woman covers her entire body and barely shows her eyes, then we disagree. If you believe a good woman should submit to her husband and is second to him instead of an equal partner, then we disagree. If you believe a good person must bow to a deity as their ruler and master, then we disagree. When I speak of an overall good it has to be seen as an overall feeling instead of an exact list of what is specifically good and bad in my view.
I will quote the words of Bertrand Russell who answered similar questions:
Since you deny ‘God’s Law’, what authority do you accept as a guide to conduct?
An Agnostic does not accept any ‘authority’ in the sense in which religious people do. He holds that a man should think out questions of conduct for himself. Of course, he will seek to profit by the wisdom of others, but he will have to select for himself the people he is to consider wise, and he will not regard even what they say as unquestionable. He will observe that what passes as ‘God’s law’ varies from time to time. The Bible says both that a woman must not marry her deceased husband’s brother, and that, in certain circumstances, she must do so. If you have the misfortune to be a childless widow with an unmarried brother-in-law, it is logically impossible for you to avoid disobeying ‘God’s law’.
How do you know what is good and what is evil? What does an agnostic consider a sin?
The Agnostic is not quite so certain as some Christians are as to what is good and what is evil. He does not hold, as most Christians in the past held, that people who disagree with the government on abstruse points of theology ought to suffer a painful death. He is against persecution, and rather chary of moral condemnation.
As for ‘sin’, he thinks it not a useful notion. He admits, of course, that some kinds of conduct are desirable and some undesirable, but he holds that the punishment of undesirable kinds is only to be commended when it is deterrent or reformatory, not when it is inflicted because it is thought a good thing on its own account that the wicked should suffer. It was this belief in vindictive punishment that made men accept Hell. This is part of the harm done by the notion of ‘sin’.
Does an agnostic do whatever he pleases?
In one sense, no; in another sense, everyone does whatever he pleases. Suppose, for example, you hate someone so much that you would like to murder him. Why do you not do so? You may reply: “Because religion tells me that murder is a sin.” But as a statistical fact, agnostics are not more prone to murder than other people, in fact, rather less so. They have the same motives for abstaining from murder as other people have. Far and away the most powerful of these motives is the fear of punishment. In lawless conditions, such as a gold rush, all sorts of people will commit crimes, although in ordinary circumstances they would have been law-abiding. There is not only actual legal punishment; there is the discomfort of dreading discovery, and the loneliness of knowing that, to avoid being hated, you must wear a mask with even your closest intimates. And there is also what may be called “conscience”: If you ever contemplated a murder, you would dread the horrible memory of your victim’s last moments or lifeless corpse. All this, it is true, depends upon your living in a law-abiding community, but there are abundant secular reasons for creating and preserving such a community.
I said that there is another sense in which every man does as he pleases. No one but a fool indulges every impulse, but what holds a desire in check is always some other desire. A man’s anti-social wishes may be restrained by a wish to please God, but they may also be restrained by a wish to please his friends, or to win the respect of his community, or to be able to contemplate himself without disgust. But if he has no such wishes, the mere abstract concepts of morality will not keep him straight.