Finding no assessment of the holistic goals for Christian spiritual development in the context of Protestant private schools, the Growing Disciples Inventory (GDI) was constructed and validated using a mixed methods approach to educational design research. To assess the 21 factors within the four Growing Fruitful Disciples curriculum processes, the GDI was developed through iterative cycles of expert review and usability testing to a final 100-item online format. GDI and exit survey responses from 595 adolescents attending eight Seventh-day Adventist schools on three continents provided strong evidence for reliability and validity. A larger international sample is recommended for further research of improved and alternate versions of this formative curriculum-aligned self-assessment tool for Christian education.