Note: Vonnegut starts talking at around the 3:40 mark.
Humanists believe that human beings produced the progressive advance of human society and also the ills that plague it. They believe that if the ills are to be alleviated, it is humanity that will have to do the job. They disbelieve in the influence of the supernatural on either the good or the bad of society, on either its ills or the alleviation of those ills.
There’s a widely disseminated Kurt Vonnegut quote that puts things even more succinctly:
I am a humanist, which means, in part, that I have tried to behave decently without any expectation of rewards or punishment after I'm dead.
It's a definition Vonnegut, Asimov’s honorary successor as AHA president, a scientist’s son, and, famously, a survivor of the firebombing of Dresden, embodied, though surely not the only one he coined.
In his 1992 acceptance speech for the association’s Humanist of the Year award, above, he recalls how a student pressed him for a definition. He chose to fob the kid off on better paid colleagues at the University of Iowa, but privately came up with another take:
…a humanist, perhaps, was somebody who was crazy about human beings, who, like Will Rogers, had never met one he didn't like. That certainly did not describe me. It did describe my dog, though.
As the title of Vonnegut’s speech implies ("Why My Dog is Not a Humanist"), Sandy, his undiscriminating Hungarian sheepdog, ultimately fell short of satisfying the criteria that would have labelled him a humanist. He lacked the capacity for rational thought of the highest order, and moreover, he regarded all humans - not just Vonnegut - as gods.
Ergo, your dog is probably not a humanist either.
Characteristically, Vonnegut ranged far and wide in his consideration of the matter, touching on a number of topics that remain germane, some 20 years after his remarks were made: race, excessive force, the treatment of prisoners…and Bill Cosby.
For introduction to humanism, please see: Stephen Fry Explains Humanism in 4 Animated Videos: Happiness, Truth and the Meaning of Life & Death