Background: Patient satisfaction with religious/spiritual (R/S) care during mental health treatment has been associated with a better treatment alliance. Aims: To investigate the longitudinal relations between (un)met R/S care needs and treatment alliance/compliance over a 6-month period. Method: 201 patients in a Christian (CC) and a secular mental health clinic completed a questionnaire (T0) containing an R/S care needs questionnaire, the Working Alliance Inventory (WAI) and the Service Engagement Scale (SES). After 6 months 136 of them took part in a follow-up (T1). Associations were analysed using hybrid linear mixed models and structural equation modelling. Results: R/S care needs decreased over time, but a similar percentage remained unanswered (e.g. 67% of the needs on R/S conversations in a secular setting). Over a 6-month period, met R/S care needs were associated with a higher WAI score (β =.25; p <.001), and unmet R/S care needs with lower WAI score (β = −.36; p <.001), which were mainly between subjects effects. Patients reporting a high score of unmet R/S care at baseline, reported a decrease in SES over time (β = −.13; p <.05). Conclusions: Satisfaction with R/S care among mental health patients is related to a better treatment alliance. When unmet R/S care needs persist, they precede a decrease in treatment compliance. Mental health professionals are recommended to assess the presence of R/S care needs and consider possibilities of R/S care especially in the first weeks of treatment.