Kurt Vonnegut gave his definition of what it means to be a humanist. Well, he gave a statement about it. He didn’t give a complete definition.
What do you suppose that he’s referencing when he says, “without the expectation of rewards or punishments after you are dead?” Several things. What he implies is that there are no consequences for what we do in this life, nothing that lasts after we die. Most humanists say that they don’t believe in an afterlife although they all know better. So, Vonnegut seems to be implying that he has no incentive for being decent other than his own inherent goodness.
That implies two other points. One, he implies that he knows the correct definition of what it is to “behave decently.” The other is that he thinks he’s able to fulfill what it is to “behave decently” without God’s goodness. His first assertion commits the moralistic fallacy. The human mind has no way to determine good, evil, right, wrong, decent, indecent, or any such thing without divine revelation. Any such self-generated assertion is a bare claim. Only God can determine what is right and what is wrong. Only God can reveal this reality to humans.
As to fulfilling what it is to “behave decently,” even after God has revealed His Laws to us, that is, His Love to us, we can’t possibly fulfill that Love without the God Who is Love.
Vonnegut also implies something about origins. Humanism requires a naturalistic (no God) origin of the universe. So Vonnegut’s statement isn’t made in a vacuum. He makes his statement under a cloud of no God in any aspect of life. This is a huge unsupported assertion, but it’s unspoken. It’s part of the word humanist.
What about those who follow Christ? If we expect that God will provide rewards and punishments, that means that we believe that God is just. It also means that actions have consequences. It doesn’t mean that we do good works to appease God or to work for our salvation. Salvation is a gift. God’s communication to every person is a gift. God’s faith is a gift. God’s grace is a gift. And righteousness is a gift from God.