First of all, it is worth noticing how the Lord addresses those listening to Him here: ‘My friends’. Not slaves, not servants, not even disciples—friends. Who are friends? For us, they are the people we love, whom we do not want to leave, for whom we wish...
First of all, it is worth noticing how the Lord addresses those listening to Him here: ‘My friends’. Not slaves, not servants, not even disciples—friends. Who are friends? For us, they are the people we love, whom we do not want to leave, for whom we wish the very best. So, when warning them, we would probably look for ways to avoid the coming danger and speak of sensible precautions... But when Jesus speaks to His friends about future calamities, persecutions and killings, He says something unexpected: ‘Do not be afraid’. Will the troubles pass them by? No. But there is no need to fear death—physical death—because the killer can do nothing more to us, and for us it is not yet the end.
So if there is anything we should take seriously, it is what separates us from the One who gives eternal life: sin. Taking it seriously means trying to avoid it here and now. For after death, resurrection awaits us, and after that, judgement, pronounced by the Judge who gave His life for our salvation. And if we are not filled with reverent awe before this Judge—what biblical language calls the fear of God—our journey with Him is cut short, in Gehenna.
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12:4 But I say to you, Fear not - Let not the fear of man make you act the hypocrite, or conceal any thing which I have commissioned you to publish.
12:5 Fear him who hath power to cast into hell - Even to his peculiar friends, Christ gives this direction. Therefore the fearing of God as having power to cast into hell, is to be pressed even on true believers.