A recidivist; one who backslides, especially in a religious sense; an apostate.(noun)
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Use "backslider" in a sentence
"Henry has been called a backslider because of the lenity of his counsels, but I cannot but think it is the Spirit of Christ that influences him."
"The "backslider" is an excellent illustration of the results of periodic renewal of impulse to right living."
"And as for the common, partial, and ignorant histories of France, written in our tongue, they generally make him a kind of backslider, who might have been a Huguenot"