To prepare students to use science knowledge in their later personal or professional lives, we must attend to what they believe it means to know and learn science (i.e., epistemology). Unfortunately, we have little understanding of how students' epistemologies shift and are stabilized as they navigate their science courses. Researchers have made intuitive arguments that many microscale epistemological messages sum over time to give rise to macro-scale understandings of knowing and learning, but we have no theoretical model for how this sum unfolds. Here, we begin to build such a theoretical model. To do so, we focus on assessments and related materials in a college chemistry course as potentially consequential sources of messages about valued knowledge products and processes. We then elicited students' evolving understandings of assessment-related epistemological messages in several one-on-one interviews conducted throughout the semester. Analysis of how three students experienced, negotiated, and responded to assessment-related messages showed that interactions with the course system stabilized a consistent, well-resolved picture of the ways of knowing and learning that counted in the focal course. Specifically, good knowledge must have specific authority-mandated features and knowledge is justified primarily via alignment with an instructor-authored key. Students found utility in different (reliable) processes for achieving the aim of authorized knowledge, and some of these differences were maintained throughout the semester. Implications for modeling students' experience with course-embedded epistemological messages over time and how this work might inform practice are discussed.