One of the most persistent arguments for belief in god centres on the necessity of an ultimate law-giver and epitome of goodness.
A softer version is seen in the genuine concern that a loss of faith will result in a corresponding loss of a moral compass - a more strident argument links the existence of good and evil with proof of the reality of god. It is often asserted that without god, moral decisions degenerate to nothing more than personal preferences and the victory of "might is right".
I want to succinctly lay out my response as an atheist, and show that a supreme being is not required for objective morality.
It is helpful to distinguish between absolute morality, objective morality and subjective morality. Christian apologists frequently conflate the first two, and secular debaters often fail to point out the difference.
Theists who disagree on everything else, are unanimous that god is perfectly good. As long as the conversation stops there, in the realm of a thought experiment, then it sounds like an attractive position. However when we try to apply that bald assertion in the real world it is found to be useless at best. In practical terms there is no reliable way to translate the claim that god is perfectly good into actual moral decisions. What we actually see is theists sanctifying ethical positions they arrived at by other means, claiming that their opinions reflect the goodness of god. For every issue there are sincere christians on both sides - equally convinced about their piety.
Given the hiddenness of god - two millennia and counting - the content of his perfect goodness could only be grasped by examining his revelation in the world, in his written word and from our inner moral sense or conscience. However when we look carefully at each of these sources of moral knowledge they are found wanting.
If the created world is assumed to be a reflection of the goodness of god then we are forced to face up to the carnage of "nature red in tooth and claw". Every creature in the history of life has perished by predation, exposure, starvation, parasitic attacks, viruses and injuries. Defender of Truth has recently spelled out the facts in an excellent OP...
At least five times in the history of the world mass extinctions have violently wiped out more than 90% of living things - difficult to reconcile with Jesus' promise that god cares for every sparrow.
We would also have to conclude that the random destruction of millions of men, women and children in natural disasters is a perfect act of love. This despite Jesus' command that we should care for the physical needs of our enemies so as to prove ourselves to be perfect just as god is perfect. If god is the epitome of morality why are his actions so immoral?
What then if we look for moral guidance not in what god does but in what he says? Here again we find an absence of support for belief in absolute goodness. If the bible was the work of a perfect god then we have to accept that - under certain circumstances - slavery infanticide, kidnap, forced marriage, brutal executions and genocide are expressions of perfect morality.
It's at this point that apologists typically change the topic to a defense of god's right to act as he chooses. Ironically the case for god being the absolute standard of goodness gets abandoned in favour of "might is right". What we are left with is not a consistent source of goodness - the same yesterday and today and tomorrow - but a capricious world of ethics by divine fiat.
The problem - sometimes referred to as the Euthyphro dilemma - was identified by Plato when he posed the question of "whether the gods love the pious because it is the pious, or whether the pious is pious only because it is loved by the gods". If god commands good things because they are intrinsically good then goodness exists independent of god and we can discover moral truth without him. If on the other hand things are good because god commands them then morality is not based on anything other than god's unpredictable whims.
What then of our inner moral sense or conscience, does it provide any basis for belief in a divine law-giver?
There are two obvious objections to this assertion. One is a practical one. As we noted earlier, christians who take opposite positions on all manner of ethical questions each claim to be humbly led by their god-given conscience. If the conscience is a result of god's nature in humans then it is a very blunt tool. The other objection to a supernatural origin of a moral compass is the conflict between the conscience and the written demands of the creator. We rightly baulk at the atrocities ordered by Yahweh. Why should our conscience be outraged by infanticide and slavery if it comes from the same god who commanded these things in his word?
The reality is that theists cherry-pick ethical statements from the bible and use then to justify their moral positions post hoc.
We could safely dispose of the words ethics and morality in these sort of conversations without losing anything useful. It all amounts to nothing more than the what we call our concerns about the way our actions affect the well-being of conscious creatures. An interesting challenge for theists is to name an example that does not fit that description - one for which Sam Harris deserves all the credit.
There are moral facts about the world. There are right ways and wrong ways to behave if our concern is to maximise well-being. This is "objective morality". This is the alternative to an imaginary absolute morality and is not a matter of mere personal preference. It provides a way to address moral questions that depends on neither the whims of a deity nor the personal preferences of the majority.
Is the harvesting of embryonic stem-cells morally wrong?
An answer from a position of absolute morality might say yes or no depending on which principles about god the theist considers most compelling. In either case the answer would likely be delivered with the certainty of being on the side of the angels. In reality the answer will be one of personal preference sanctified post hoc by an appeal to selected bible principles. An answer from a position of objective morality would begin by wrestling with the possible consequences of the decision, including the potential benefits of stem-cell research and a consideration any negative effects of destroying early-stage embryos. The answer would be provisional pending further information.
It is this need to accept uncertainty that seems to worry theists. It opposes the frequent appeals in scripture to trust and obey like little children. Morality is difficult and messy. Sometimes there is no clear answer. The solution is not to be found in taking refuge in false, infantalising certainties.
There is a lot more to be said about the case for a secular foundation of morality but this OP is too long already. I recommend Sam Harris' "The Moral Landscape".