Another way of putting it is to say that humanists believe we can live good and worthwhile lives guided by reason and compassion, rather than religion or superstition, and that there are more things that unite humanity than divide it. Or, as we say in Scotland, "We're a' Jock Tamson's Bairns" - we're all the same under the skin.
Some humanists are atheists; others are agnostic. Not everyone who chooses to have a humanist ceremony will be either. I've married couples where one partner was Catholic, or Muslim or Protestant. They chose a humanist ceremony because they felt they could relate to the values of humanism, which are universal.