Right and Wrong
Many religious people believe that God is the only source of morality -- that only by referring to God can we know right from wrong. Therfore, non-religious people must be wicked.
The kind of ethics found in the Old Testament is what the pedants call deontological, that is, rule-based ethics -- a list of thou shalts and thou shalt nots. The Ten Commandments are among the most famous such lists in the Old Testament -- but watch out -- there is more than one version. However, there are many more rules in the Old Testament of course. One which is a source of extreme contention in society right now, referenced recently by my friends here, is the condemnation of homosexuality. Christian conservatives pluck the rule that if a man lies with a man as with a woman, it is abomination, from among many other rules which they ignore. For example, a passage in Deuteronomy commands the Hebrews to commit what we today call rape, ordering them to take possession of the women of their defeated enemies. In Leviticus, God commands that people who perform work on the sabbath should be stoned to death, and that people with certain symptoms of skin disease should be driven out into the desert to die. The Bible most certainly does not define marriage as between one man and one woman. On the contrary, men may have as many wives as they can afford, and widows are commanded to marry their brothers in law, no matter if there are already other wives. Men may also keep concubines, that is, sex slaves.
The Gospels have Jesus largely replacing the deontological ethics of the Old Testament with ethics based on what are called principles. Rather than lists of specific commands for actions in specific situations, these are goals or aspirations which should guide our decisions about what to do in general. Matthew famously has Jesus saying that there are only two rules, to love God and love thy neighbor (which presumably does not mean only the family next door but whoever happens to be within one's sphere of action).
Humanists are generally not inclined to deontological ethics. Some have attempted systems of so-called utilitarian ethics, in which we try to order affairs to as to bring about desired outcomes. These are generally considered to have fallen short, however, for two main reasons. First of all, we don't have a crystal ball: it is usually impossible to foresee all of the consequences of our actions, or to analyze complicated social situations well enough to know how to go about maximizing "utility." More fundamentally, we need a set of principles to decide which outcomes are desirable, and how to prioritize them when they come into conflict, so we can't even get to utilitarianism without passing through principles.
It turns out that principles are characteristic of human beings. There are many basic principles that the vast majority of people will readily accept, and that is because evolution made them part of our nature. The selective advantage of our principles derives from the essentiality of society to human life and reproduction. We succeed only by working together, sharing, and helping each other. If we did not do these things, our clawless extremities, dull teeth and slowness afoot would doom us.
In practical application, ethical reasoning is perhaps most advanced in the field of medicine. There, people have generally come to accept the primacy of four principles, which are called justice, beneficence, non-maleficence, and respect for persons (or autonomy). Justice means treating people fairly. That does not necessarily mean treating everyone alike, as people may have different needs, and may, according to some other principles, not be equally deserving (although in medicine, physicians are usually enjoined from making the latter judgment if at all possible). Beneficence means trying to help people. Non-maleficence means not hurting people. This may seem redundant with the second principle, but it isn't entirely so, and most important, it is sometimes helpful in resolving conflicts between the second principle and the first and fourth. Autonomy means that people control their own persons, and have the right to make decisions about what should happen to them.
A fundamental requirement for applying these principles is deciding who or what constitutes a person -- a moral agent, with the status to lay claim to the protection of the principles. That discussion, which is a source of much discomfort in the modern world, I will save for later.
For now, I will just say that these principles are perfectly satisfactory to almost everyone who learns of them, without reference to religion or God. They are also consistent with the principles expressed by Jesus in the Gospels, but they are radically inconsistent with much of the deontological content of the Old Testament. That is why Richard Dawkins, a famous evolutionary biologist and committed atheist, once suggested printing up T-shirts reading "Atheists for Jesus." Upon reflection, it is not at all absurd.