What does John's Gospel say about the election and security of the believer?
Of all the gospels, John goes into greatest detail and most clearly describes the election and the security of the believer. Among the texts that are most famous in this respect are 6:39. Jesus, speaking again: "This is the will of Him who sent Me, the Father, that I shall lose none of all that He has given Me, but raise them up at the last day." And again, in 10:29, "My Father who has given them to Me is greater than all; no one shall snatch them out of My Father's hand." In 15:16 we read that the disciples did not choose Jesus but He chose them. And more ominously, Judas' betrayal in 17:12 demonstrates that he was not elected to salvation: "Christ protected all that the Father gave Him, but the betrayer was the one doomed to destruction."
Yet, balancing this emphasis on God's electing choices, is John's equally significant emphasis that believers must choose to abide, or remain, in Christ (particularly in John 15). They must remain in Him so that they can bear much fruit (v. 4). But this is not simply remaining in order to experience Christian maturity, as opposed to staying in an immature Christian state. Verse 2 offers the solemn warning that the Father cuts off every branch that bears no fruit. It is not acceptable to simply refer to this as the same as the pruning that He describes in this context, because cutting off branches may allow the rest of a plant to continue to grow or indeed grow better. But it means the death, the complete end of existence for the branches cut off.
John's own harmonization of these apparently contradictory lines of thought appears in his first letter (1 John 2:19). There are people who do abandon what they previously have professed with respect to Christian faith: that is, abandon it altogether, leave Christ, leave His Church and follow some other contradictory, incompatible, false teaching. This took place in Ephesus not long after John's gospel was written and John replies to it in 1 John 2:19. He describes such people, saying: "They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained. But their going showed that none of them belonged to us."
Yes, there are promises for security of the believer throughout the New Testament and particularly in John's writings. But the security and assurance of salvation are present only as people remain in the vine – continue believing in Jesus, remain connected to Him. If they do not, it demonstrates that they never really truly were connected. But it means also that, short of repentance, they will be lost. So with John's precious promises must be also be paired his equally crucial warnings.