Previous longitudinal research indicates that although religion may affect how personality traits are expressed, religion does not affect people's underlying personalities. However, such research has drawn from small North American samples and relatively short time intervals that do not include data from individuals prior to conversion. Here, we use a representative national sample of New Zealand adults over 9 years (2009–2017, N = 31,604) and piecewise latent growth models to assess longitudinal change in Big Five personality and Honesty-Humility before and after conversion to/deconversion from Christianity ( N = 540 converts, N = 886 deconverts). We observed no personality changes before conversion or after deconversion. However, we observed increases in Honesty-Humility, Conscientiousness, and Neuroticism following conversion. We also observed increases in Honesty-Humility and decreases in Agreeableness preceding deconversion. These findings indicate that religious conversion initiates specific changes in character, the most pronounced of which relate to increases in modesty and greed-avoidance.