I think it is Christ's way, as seen throughout his word, of warning those who have entered into covenant relationship with him that they can fall away and warning them not to fall away. In John 15, he is exhorting and warning all those listening to him, including his closest disciples, to continue in the faith.
Those biblical warnings and exhortations in both the OT and the NT are the means God uses to keep his elect, and cause them to persevere in the faith until the end. Christ really does say in John 15 that there are those in him who will be examined for fruit and found fruitless, and so will be cut off. This makes a good argument for the Presbyterian view of the new covenant, as has been discussed.
Those biblical warnings and exhortations in both the OT and the NT are the means God uses to keep his elect, and cause them to persevere in the faith until the end. Christ really does say in John 15 that there are those in him who will be examined for fruit and found fruitless, and so will be cut off. This makes a good argument for the Presbyterian view of the new covenant, as has been discussed.