Theophylact of Ohrid

"Do not be afraid," He says, "of those who kill the body" and can do nothing more to harm you. For the harm from those who injure the body does not amount to much. The body will suffer what is proper to it even if they do not harm it. But one should fear Him who punishes not only the body but also the soul, subjecting an immortal being to endless torments, and moreover in fire. In this way Christ trains His friends in spiritual courage, makes them witnesses, and drives away from them the fear of man. …

Commentary on Luke ↗

Bede

Fear him who, after he has killed, has the power to cast into Gehenna. For there are two kinds of persecutors: one openly raging, the other deceitfully and fraudulently flattering: wishing to arm and instruct us against both, the Savior commands us above to beware of the hypocrisy of the Pharisees, and here not to fear the slaughter of executioners, because evidently after death neither the cruelty of the former nor the deceit of the latter is able to endure. …

On the Gospel of Luke ↗

Bonaventure

It is necessary that we fear God. Whence in the Gospel: "Do not fear those who kill the body and after this have nothing more that they can do; but I will show you whom you should fear: fear him who, after he has killed, has power to cast into gehenna." I would prefer to be in the greatest punishment of this world for seven thousand years than to endure the least eternal punishment. The Apostle: "It is a fearful thing to fall into the hands of the living God," because God afflicts for eternity. …

Collationes de Septem Donis, Collation 2 ↗