弃暗投明 abandon darkness and embrace the light
Explanation
离开黑暗，投向光明。比喻在政治上脱离反动阵营，投向进步方面。也泛指从坏的方面转向好的方面。
Leaving darkness and turning to light. It is a metaphor for leaving a reactionary camp and joining a progressive side politically. It also generally refers to changing from bad to good.
Origin Story
话说汉朝末年，天下大乱，群雄逐鹿。刘备手下大将关羽，本是跟随曹操，因看不惯曹操的奸诈残暴，又敬佩刘备的仁义，便在一次战役中，瞅准时机，带着自己部下，上演了一出“单刀赴会”的好戏，顺利投奔刘备，从此为刘备效力，为蜀国鞠躬尽瘁，留下了忠义的佳话。关羽的这一举动，便是弃暗投明，投向光明，从此名扬天下，成为人们敬佩的对象。
In the late Han Dynasty, the country was in chaos, and warlords were vying for power. Guan Yu, a high-ranking general under Liu Bei, was originally under Cao Cao. Because he couldn't stand Cao Cao's treachery and cruelty, and admired Liu Bei's benevolence, he seized the opportunity in a battle to lead his troops to a dramatic ‘single-knife meeting,’ successfully defecting to Liu Bei. From then on, he served Liu Bei faithfully and devotedly, leaving behind a story of loyalty and righteousness. Guan Yu's action is a classic example of "abandoning darkness and turning to the light," earning him fame and admiration.
Usage
作谓语、宾语；比喻改邪归正，或指政治上从反动阵营投向进步阵营。
Used as predicate or object; a metaphor for changing from evil to good, or politically switching from a reactionary camp to a progressive one.
Examples
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他最终弃暗投明，加入了革命队伍。
tā zuìzhōng qì àn tóu míng, jiārù le gémìng duìwǔ。
He eventually abandoned the darkness and joined the revolutionary ranks.
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经过一番思想斗争，他终于弃暗投明，重新做人。
jīngguò yīfān sīxiǎng dòuzhēng, tā zhōngyú qì àn tóu míng, chóngxīn zuòrén。
After a mental struggle, he finally turned to the light and started a new life.
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在历史的转折点，许多人毅然弃暗投明，投身革命洪流。
zài lìshǐ de zhuǎnzhé diǎn, xǔduō rén yìrán qì àn tóu míng, tóushēn gémìng hóngliú。
At the turning points of history, many people resolutely abandoned the darkness and joined the revolutionary torrent.