In the world but not of it
The disciples will certainly need God’s protection, for both the world (14) and the devil (15) will be their enemies. The word world (kosmos) occurs 17 times in this prayer, including 7 times in this short section. Jesus has already told them that, like him, they don’t belong to the world and so they too will incur hatred and opposition from it (14, 16; 15:18-19). That will be tough so he prays particularly for them about that.
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His prayer is not that they will be removed from the world, but that they’ll be protected in it (15). Heaven is a safer place, both for Jesus and for his followers, but that’s not where we are yet meant to be. God loves the world and wants to save it which is why Jesus came (3:16-17). And just as Jesus was sent into the world by the Father, so too we are sent into the world by Jesus (18; 20:21).
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As Christians, we are to be sanctified – set apart for God and living according to his word (17). That should not lead to keeping a safe distance from the world but to loving service in it, just as it did for Jesus. So Jesus again dedicates himself to doing God’s will, in order that we might be a holy people (19), able to shine God’s light in a dark world (Matt.5:14-16; Phil.2:15-16).