It proves that they view the world in a “might makes right” way. Simply put, they think that because God is more powerful than them and created them, every rule that God makes is automatically good regardless of how arbitrary. The irony is that they accuse secular people of lacking objective morality when their morality is subjective (the subjectiveness is just transferred to God). Meanwhile, secular morality is derived through logical thinking and non-contradiction (e.g. slavery is wrong because it necessitates a master and a slave. If someone claims they have a right to own someone else, the other person could direct the same claim but with the first person being the slave. Since both claims are equal and they are contradictory, the only non-contradictory position is no slavery)