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.
可以想成這樣一個畫面：一個人指著某一天說，在那之前的我是另一個人。那天之後的日子，他自己算成第二次出生。born-again 講的就是站在那條線另一邊的人。跟著這個畫面一起來的，還有一股熱度——剛換了一個人的人，通常也很想拉別人一起換。這股熱度就是這個詞後來被借去開玩笑的原因。
From the word’s history
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, "born-again"