When we think of objective morality, we think of a moral code that from outside ourselves - that is, it's not reliant on the collective opinions of human beings, but is transcendent of that. And as Christians, we believe that morality is grounded in God.
However, many atheist argue that even if God exists, morality would not be objective. "How is that?" you may ask. Well, it depends on what we mean be objective and subjective.
When something is subjective, it means it depends on the subject (in this case, us) for it's meaning. So, if I say chocolate ice cream is the best, it's a subjective opinion as it's based on the subject, me. When we say something is objective, we mean that it's mind independent. It's doesn't rely on opinion to determine it's truth. So, if we say objective morality is dependent on God to determine it's truth, atheists will argue that it then is not really objective as it depends on a mind (God) and is therefore subjective. It is here where 2 misunderstandings arise both for the theist and the atheist.
We need to be careful when we use the term "Objective Morality". Dr. William Lane Craig purposefully uses the term "Objective Moral Values and Duties". This is because those abstract objects - the values and duties - even if created by God have an intrinsic value unto themselves. For example, if God created green grass, the grass would still be green in and of itself no matter if an intelligent mind were behind it. The grass is still objectively green. The same would be said for moral values and duties. The values and duties have intrinsic moral distinctions in and of themselves. So rape is still wrong objectively even if God determined that truth which brings us to our next misunderstanding.
When we say God is the standard for morality, we aren't simply stating that God decided this was right and this was wrong. We are saying God Himself is the standard - that is - His character is the standard for determining moral truths. It is not as if there is a higher moral standard that God ascends to as that would make that standard God. It is for this reason that the infamous Euthyphro Dilemma fails. "Is the pious loved by the gods because it is pious, or is it pious because it is loved by the gods?" It is odd that Plato's question posed to the Greek gods is then posed to God, but it is a fair question. However, it's a false dichotomy as God's character is the standard for moral truth. So something that aligns with God's character is good while anything that departs from His character is evil.
Now we have seen that in fact, moral values and duties are objective and the argument against this fails.