Claiming that you need a belief in God in order to be moral is a really bad argument — not only because it’s untrue, but because it’s the fastest way to smother a potentially productive conversation with an atheist. If you want to persuade an atheist, or want to have any kind of conversation at all, you’re going to need to leave this argument at the door.
It’s a bad argument primarily because it’s almost always untrue. Many atheists are atheists precisely because they are moral people. They reject the idea of God or a particular god (like the Christian god) on moral grounds, such as the argument from evil (which requires some sort of moral system to work) or because of moral problems in the Christian Scriptures. There is a difference between saying that a person has no valid reason for being moral and that they are not in fact moral; just because there are no valid reasons for being a Florida Gators fan doesn’t change the fact that many people are, unfortunately, Gator fans.
But even attempting to make this distinction may be counterproductive, because it’s a very fine one and is likely to be misunderstood. Morality is an incredibly personal subject; you have no right to be surprised when someone gets offended when you insinuate, even unintentionally, that they have no ethical standards. It’s possible that you may encounter radicals who openly embrace moral relativism, but despite what your Christian apologetics training told you, these people are relatively rare. Go into discussions assuming your atheist debate partner has ethical principles which he or she values. It’s a much safer assumption.
Claiming you need to believe in God to have morality fails on all three rhetorical planes: it’s observably untrue, it pisses people off, and, as a consequence, you will lose credibility with the atheists you’re talking to. Unfortunately, it’s the argument many Christians choose to lead with.