If someone tells me, he thinks that belief in a god is necessary to be good, I would politely disagree with him, but I would not be offended or angry. One thing I have learned though is that many Christians wonder what is the point for non-believers in a god, to mention they don't believe in one.
After all, if atheists are right, what harm is it to let Christianity in the U.S. or Islam in the Middle East or Hinduism in India, dominate? Well, first of all, while Christians don't believe in Islam, if they are a woman in parts of Islamic dominated society, they are forced to wear veils.
That is the consequence of a belief they don't believe in. Religion because it happens not to be true, does not lose its' power and influence over a society. Atheists and humanists don't urge for laws based on reason based thinking because they worry about the god of a society, but because they want to live in a society where they aren't restricted based on the religiou views of others.
For example, in the state of Alabama it is illegal to sale a sex toy, because of religion. There are still obscenity laws and prosecutions on what adults can view of films made by and with, other consenting adults, with support for these laws driven by religion. Gays can't marry who they desire to marry and who they are in love with, based on religion.
Not that long ago in the state of Georgia, two consenting adult men who had sex with each other, could be prosecuted. Alcohol is restricted in Georgia, on Sundays. I could go on and on. Humanists believe a nation founded upon reason and the Enlightenment should fully live up to those ideals.
We should not punish our citizens if they cause no harm, other than to the religious beliefs of a majority. If someone wants to live under religious doctrine, good for them, but I do not wish to be included. So, if I or someone else states one cannot be good without a belief in god, it more than just obvious, because there are many people who are good without a belief in a god. It is also a statement that to follow the doctrines of a religion does not necessary mean one is following good moral law.
Morality must be reason based, even if from the mouth of a god, or it is arbitrary. Even a god, must use reason. Reason is our tool to examine any and all issues and ideas before us. Even if someone disagrees with me, that is one thing, it is the suppression of the message in the first place that most concerns me.