"...so that you will not go astray"
Why does Jesus warn them so clearly that they will be persecuted? It’s because the greatest danger they face is not the persecution itself but the danger they’ll be tempted to give up because of it. “All this I have told you so that you will not go astray” (1). Peter is about to deny Jesus, the disciples will be scattered (13:38; Mark 14:27-31) and there’ll be opposition ahead, bringing the temptation to renounce Jesus.
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The first Christians were all Jews for whom expulsion from the synagogue was a distressing price of following Jesus (2; 9:34-35). Even worse, just as some Jews had believed that Jesus deserved to be put to death as a blasphemer (8:19), they will likewise think that killing Christians will be offering service to God. In this they were sincere, but they were sincerely wrong. Once again, Jesus emphasises that their blindness is due to their not knowing either the Father or the Son (3; 8:19; 14:7; 15:21).
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But to be forewarned is to be forearmed (4). He tells them all this “now before it happens” (13:19; 14:29) so that when it does happen it will confirm their faith. By keeping focused on Jesus and his words, we are able to stand firm to the end (Mark 13:13; Heb.12:1-3).