Feng Yi once served as an official under Wang Mang's Xin dynasty. When Liu Xiu raised the banner of rebellion, Feng Yi observed that Liu Xiu harbored great ambitions and was no ordinary man in speech and conduct, so he decided to defect to Liu Xiu's side.
Feng Yi was a man of remarkable humility who never boasted of his achievements. When walking on the road and encountering other generals, he would immediately step aside to let them pass. He conducted all matters with order and discipline, following clear rules. Whenever the army camped for the night, while other officers gathered to discuss merits and rewards, Feng Yi would sit alone under a tree, quietly deep in thought. The soldiers came to call him "General of the Big Tree." After the capture of Handan, when the troops were reorganized, every soldier declared they wished to serve under the "General of the Big Tree." Emperor Liu Xiu held Feng Yi in the highest regard because of this.
At the time, Emperor Gengshi sent Li Yi, the Prince of Wuyang, leading an army boasting 300,000 troops, along with Wu Bo, the Governor of Henan, to jointly defend Luoyang. Emperor Guangwu appointed Feng Yi as General of Mengjin, commanding forces stationed by the Yellow River, directly opposing Li Yi and his men.
Feng Yi wrote to Li Yi, saying, "I have heard that a bright mirror can reflect one's image, and reflecting on the past can illuminate the present. In ancient times, Weizi, the elder half-brother of King Zhou of Shang, left Shang to join King Wu of Zhou; Xiang Bo, the youngest uncle of Xiang Yu, rebelled against Chu and defected to the Han king; Zhou Bo deposed Emperor Shao because he was not Emperor Hui's son and welcomed Emperor Wen; Huo Guang deposed Prince Changyi for his cruelty and welcomed Emperor Xuan. These men followed the will of Heaven, aligning with the principles of survival and decline, rise and fall, thus achieving fame that lasts for ages. Now, even if the Gengshi Emperor can still be supported and barely survive, you are far from him, isolated in a corner—how long can you hold out? Today, Gengshi is cruel and unjust, the Red Eyebrows are at the gates, the nobles and ministers harbor treacherous intentions, the court's discipline is shattered, war flames rise from all directions, and heroes gather like clouds. If you can see clearly what is right and wrong, success and failure, and quickly decide your allegiance, your merit can rival that of Weizi and Xiang Bo, turning danger into safety and disaster into fortune—this moment is crucial. But if you wait until our army charges in and surrounds the city, then it will be too late to regret!"
Li Yi received the letter, weighed down with worry. He recalled how he and Liu Xiu had once sworn like brothers to restore the Han dynasty, but after Emperor Gengshi took the throne, Li Yi turned and conspired with him to frame Liu Xiu. Now, with Gengshi's regime on the brink of collapse, Li Yi wondered: if he surrendered, would Liu Xiu ever forgive him? So he wrote back to Feng Yi, pouring out his inner turmoil.
After Li Yi and Feng Yi exchanged letters, they ceased hostilities, allowing Feng Yi to capture several cities in quick succession. Emperor Liu Xiu repeatedly praised Feng Yi's exceptional strategy, remarking, "Your single letter is worth ten thousand soldiers."
Later, the idiom "Fear Heaven, Know Fate" came to describe revering heaven's will and knowing when to submit, symbolizing an understanding of the times.
Source: *Book of the Later Han*, "Biography of Feng Yi"
Meaning of the Idiom: Later, the Chinese idiom "畏天知命" came to describe revering heaven's will and knowing when to submit, symbolizing an understanding of the times.