to bring something back into someone's mind: to remind a person of what they already know
Greek has a small family of memory words, and this one is specifically about causing memory in someone else. The plain verb for remembering (μιμνήσκω) describes what happens inside a person, and another common verb (μνημονεύω) describes keeping something in mind over time. This verb describes the work of whoever does the prompting, which is why nearly every use of it sits in a letter or a speech, with a writer or a leader doing the prompting.
What gets brought back is almost never new information. The writers reach for this word precisely when their readers already know the material, and they say so out loud:
- Jude tells his readers he wants to remind them, though they know it all already
- Peter promises to keep raising the same points with people already settled in the truth
- Timothy is told to keep putting these things in front of the church
Reminding, in this sense, is not filling a gap in someone's knowledge. It is keeping known truth in active use.
Two uses stand out from the rest. In John 14:26 the one doing the reminding is the Holy Spirit, who will bring back to the disciples everything Jesus said, tying their later testimony to words they had actually heard rather than to fresh revelation. In 3 John 1:10 the word turns sharp: the writer promises to bring up what Diotrephes has been doing, out loud and in front of the church, which is nearer to raising a charge than to jogging a memory.
The matching noun (ὑπόμνησις) names the reminder itself, and 2 Peter uses both, describing the whole letter as an effort to stir people up by way of reminder.