John Calvin Commentary John 17:15

"I pray not that thou shouldest take them from the world, but that thou shouldest keep them from the evil [one]." — John 17:15 (ASV)

I ask not that you should take them out of the world. He shows in what the safety of believers consists: not that they are free from every annoyance and live in luxury and at their ease, but that, in the midst of dangers, they continue to be safe through the assistance of God.

For he does not admonish the Father about what should be done, but rather makes provision for their weakness, so that, by the method which he prescribes, they may restrain their desires, which tend to go beyond all bounds. In short, he promises to his disciples the grace of the Father: not to relieve them from all anxiety and toil, but to furnish them with invincible strength against their enemies, and not to allow them to be overwhelmed by the heavy burden of contests which they will have to endure.

If, therefore, we wish to be kept according to the rule which Christ has laid down, we must not desire exemption from evils, or pray to God to convey us immediately into a state of blessed rest, but must rest satisfied with the certain assurance of victory, and, in the meantime, courageously resist all the evils, from which Christ prayed to his Father that we might have a favorable outcome.

In short, God does not take his people out of the world, because he does not wish them to be weak and slothful; but he delivers them from evil, so that they may not be overwhelmed; for he wishes them to fight, but does not allow them to be mortally wounded.