Adam Clarke's Commentary on the Bible

Verse 28 (Matthew 10:28)

Fear not them which kill the body - Των αποκτεινοντων . Those who slay with acts of cruelty, alluding probably to the cruelties which persecutors should exercise on his followers in their martyrdom. But are not able to kill the soul. Hence we find that the body and the soul are distinct principles, for the body may be slain and the soul escape; and, secondly, that the soul is immaterial, for the murderers of the body are not able, μη δυναμενων , have it not in their power, to injure it.

Fear him - It is, not hell-fire we are to fear, but it is God; without the stroke of whose justice hell itself would be no punishment, and whose frown would render heaven itself insupportable. What strange blindness is it to expose our souls to endless ruin, which should enjoy God eternally; and to save and pamper the body, by which we enjoy nothing but the creatures, and them only for a moment!

- Adam Clarke's Commentary on the Bible