metaphor
An oxymoron is a phrase of opposites - there are no opposites in the lines. Flashback is an interruption in narration to tell something that had happened in the past. It's often used for clarification or character development. This is not a flashback. Personification is giving nonhuman things humanlike traits. The lines are actually talking about humans, not animals. This is why metaphor is correct. The priest doesn't actually have a flock of sheep. The flock that is mentioned are his parishioners (people who attend religious services). He says that they were either in bed or had died.