apostatize

(ə-pŏs'tə-tīz')
intr.v., -tized, -tiz·ing, -tiz·es.
To abandon one's religious faith, a political party, one's principles, or a cause.

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verb

To abandon one's cause or party usually to join another: defect, desert, renegade, tergiversate, turn. Slang rat. Idioms: change sides, turn one's coat. See approach/retreat, trust/distrust.