Teachers' English language ideologies play a crucial role in English language education reforms. It is important to understand how their ideologies evolve outside and inside the classroom. This paper explores how micro-level factors shape teachers' English-only ideology, encompassing both their beliefs and their associated pedagogical practices. Adopting a Vygotskian sociocultural theory and activity theory perspective, this case-study research focuses on two Indonesian EFL teachers. Data include interviews, classroom observations, and relevant artefacts. Data analysis follows a combined genetic and activity system analytical framework. Findings revealed that when driven by a tension between their English-only ideology and the students' varied levels of English competency, both teachers compromised to their practices, reflecting transformations in their ideology. Beyond uncovering the process of changes in EFL teachers' English-only ideology, this paper offers a new and contingent perspective on the relationship between teachers' ideological belief, practice, and context.