Matthew 10:28
28 And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
The Grace New Testament Commentary - Matthew 10:28
10:28 They are to fear God, not their human persecutors, since their persecutors had less authority over the total person. People could "kill the body" and not any more than that, but God could "destroy both soul and body in hell." The prospect of destruction does not mean the apostles would go to hell if they were unfaithful. It simply means that in the end only God's disposition of affairs matters. And their enemies will face that fearsome higher authority when the time comes for believers to be vindicated.
Thomas Constable's Expository Notes on the Bible - Matthew 10:28
10:28 Good news also helps to conquer fear, if the disciple will remember that the worst that a human adversary can do does not compare with the worst that God can do. Jesus was not implying that true believers might go to hell if they do not remain faithful to God. His point was that God has power over the disciple after he dies, whereas human adversaries can do nothing beyond killing the disciple's body. The believer needs to remember that he or she will stand before God one day to give an account of his or her stewardship.