Dictionary Definition :
Definition[1]
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This refers to the state of faith in the worshipper; the three 不 are impure, not single, not constant; the three 信 are the opposite.
Source
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A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous
Definition[2]
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(名數)梁曇鸞就念佛者立三不三信:一信心不淳,二信心不一,三信心不相續。是曰三不。反之則曰三信。淨土論註下曰:「一者信心不淳,若存若亡故。二者信心不一,無決定故。三者信心不相續,餘念間故。此三句展轉相成。以信心不淳,故無決定。無決定故,念不相續。亦可念不相續,故不得決定信。不得決定信,故心不淳。」西河道綽承之,更闡明三信。安樂集上曰:「若能相續,則是一心。但能一心,即是淳心。具此三心,若不生者,無有是處。」文類聚鈔行卷曰:「三不三信誨慇懃,像末法滅同悲引。」
Source
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丁福保《佛學大辭典》