Wasn't Sir Thomas More a humanist, and a Catholic? If a person can't be a humanist and believe in god, then I think you're going to have to throw away a huge lot of humanist writings and ideas.
I think we are getting caught up on the differences between humanism and secular humanism.
My (not very well read) example would be someone who believes that we shouldn't kill each other because we all share the "human condition" can also believe that God told us "Thou shall not murder."