BackgroundMedical advances have led to new challenges in decision-making for parents of seriously ill children. Many parents say religion and spirituality (R&S) influence their decisions, but the mechanism and outcomes of this influence are unknown. Health care providers (HCPs) often feel unprepared to discuss R&S with parents or address conflicts between R&S beliefs and clinical recommendations. Our study sought to illuminate the influence of R&S on parental decision-making and explore how HCPs interact with parents for whom R&S are important.MethodsA longitudinal, qualitative, descriptive design was used to (1) identify R&S factors affecting parental decision-making, (2) observe changes in R&S themes over time, and (3) learn about HCP perspectives on parental R&S. The study sample included 16 cases featuring children with complex life-threatening conditions. The length of study for each case varied, ranging in duration from 8 to 531 days (median = 380, mean = 324, SD = 174). Data from each case included medical records and sets of interviews conducted at least monthly with mothers (n = 16), fathers (n = 12), and HCPs (n = 108). Thematic analysis was performed on 363 narrative interviews to identify R&S themes and content related to decision-making.ResultsParents from 13 cases reported R&S directly influenced decision-making. Most HCPs were unaware of this influence. Fifteen R&S themes appeared in parent and HCP transcripts. Themes most often associated with decision-making were Hope & Faith, God is in Control, Miracles, and Prayer. Despite instability in the child’s condition, these themes remained consistently relevant across the trajectory of illness. R&S influenced decisions about treatment initiation, procedures, and life-sustaining therapy, but the variance in effect of R&S on parents’ choices ultimately depended upon other medical & non-medical factors.ConclusionsParents consider R&S fundamental to decision-making, but apply R&S concepts in vague ways, suggesting R&S impact how decisions are made more than what decisions are made. Lack of clarity in parental expressions of R&S does not necessarily indicate insincerity or underestimation of the seriousness of the child’s prognosis; R&S can be applied to decision-making in both functional and dysfunctional ways. We present three models of how religious and spiritual vagueness functions in parental decision-making and suggest clinical applications.Electronic supplementary materialThe online version of this article (10.1186/s12904-018-0360-y) contains supplementary material, which is available to authorized users.