Ordinary Time 2023: Falling Sparrows, Numbered Hairs, and the Dread of Gehenna

Jesus says that we should not fear those who revile us or even those who can only kill the body. Jesus says that the Father's care for us is so deep that we need not fear. But Jesus also says we should fear the one who can destroy both body and soul in Gehenna. What does this mean and who is Jesus talking about?