One way to hold this word: a person can point at a date and say that the life before it belonged to someone else. Everything after that day is counted as a second life, a second birth. Born-again is the word for a person standing on the far side of that line — and for the heat that comes with standing there, because someone who has just started over usually wants everyone else to start over too.
The phrase comes from the New Testament: 'no one can see the kingdom of God without being born again' (John 3:3). The second birth in that line is the picture the modern word still carries.
這個說法出自《新約·約翰福音》三章三節：人若不重生，就不能見神的國。那句話裡的「再生一次」，就是今天這個詞還在用的畫面。
Wiktionary entry for born-again, etymology citing John 3:3 (Today's New International Version) — kaikki:3f6f69a190036ae4:1338755201:9765
Two branches
信仰上的重生
這一組直接就是那個畫面本身：一次強烈的信仰經驗，把一個人的人生切成前後兩段。
adjective · B2
Used in born-again Christian to describe someone who has had one strong religious experience and now follows an evangelical kind of Christianity, speaking of that experience as the start of a completely new life.
重生的基督徒
剛改變就到處勸人的熱度
把同一個畫面搬到日常習慣上：一個人戒了什麼、開始了什麼，於是也有了「重新做人」的那股勁。借的是熱度，不是信仰，所以語氣是調侃的。
adjective · C1
Used in front of words like non-smoker, vegetarian or cyclist to describe someone who has just given something up or taken something up, and who now talks about it constantly and keeps suggesting that other people do the same.
剛改變就熱心勸人的