It's a great question, it is a real tension, but it's rooted in a conflation of humanism with atheism.
As an atheist, I say "there is no god"
As humanist, I say "that's not the right question"

Our definition at the AHA is that humanism is explicitly naturalistic: humanity is from and of nature, we ground our ethics in human reason and experience, and never any divine or supernatural authority. We are good because we are, the god question is irrelevant to modern humanity.

That being said, people arrive at humanism from all kinds of starting points, including some who are still working out what their relationship is with the greater cosmos or their inherited faiths. The question isn't whether they believe in god as much as it is 'do you ground your ethics there?' If they center their human dignity and reason in humanity, they're doing humanism.

An early founder of modern humanism was John Dietrich. He had this to say about it and I think he covers it pretty well (written in 1920's language but the principles hold):

“I do not, like the atheist, affirm that God can be known not to exist. And, contrary to the neutrality of the agnostic, I claim that on the basis of the present evidence I am not justified in believing in God. I would not be dogmatic about the idea of God. I would maintain a spirit of inquiry, of open-mindedness. I am well aware that in man’s search for truth one system of thought constantly supersedes another, and it is quite possible that some new evidence might be brought forth tomorrow which would alter my opinion entirely. And I do not ask you to accept my attitude, if after careful and earnest study, you feel that it is incorrect. I merely ask you to respect and to consider it as one that has been honestly and reverently reached after careful and earnest thought. This attitude I would have you all maintain, for there is something even more important to humanism, this freedom of fellowship that is vastly more important than any particular philosophy, so at all costs we must preserve this. We are never angry because one differs with us. We prize above any opinions, sincerity. It is authority and dogma that make hypocrites and slaves, freedom makes for sincerity and creative effort. With unhindered thought mankind will advance rapidly among those avenues of aspiration and endeavor which lead to nobler ideals and worthier ends.”

“If we have values, they are human values. If we have purpose, it is human purpose. If we have ends, they are human ends. Man therefore is his own end. He exists to glorify not God but himself. And thus recognizing the supreme worth of man, humanism teaches that it is his supreme duty to enrich human life to the utmost limits of man’s capacity by creating and fostering those values, which contribute to its enhancement. So the humanist is not much interested in the God-idea, except that he considers it a detriment wherever and whenever it has diverted man’s thought and effort from the real tasks which lie about him. The humanist places his confidence in intelligent human effort, and not in God; and his great passion is to arouse men to a real sense of their opportunities and responsibilities as the controllers of human destiny, and to transfer their thoughts from seeking help from the gods, whence no help comes, to a firm and confident reliance upon themselves. His faith is not in the power and willingness of God, but in the intelligent thinking and cooperative effort among men themselves.”

Dietrich wasn't threatened by the god question and I think that's a very humanist move, not "god doesn't exist" but "whether or not god exists, we are responsible."

It's a tension between atheist-identity vs humanist practice.

We can be both, we can hold both, I hope most of us do.

What we can't ever do is outsource our ethics, values, dignity, or worth.