The affective domain exercises significant influence over the learning of mathematics and, therefore over the future praxis of teachers. This is why the affective domain needs to be included in teacher training. Therefore, our aim is to show how the praxis carried out in the classroom by teacher educators promotes the evolution of beliefs about mathematics and thus that their specialist knowledge—specifically, the KFLM subdomain—could change. We used a Likert-type questionnaire divided into three sections with four response options, answered by 59 teachers. The results show the evolution of some student beliefs towards a higher percentage in favor and the stability of others depending on the praxis implemented by the teacher educator. The favorable evolution is related to the beliefs about the role of teachers in their training and their own learning. This could promote a change in the KFLM of future teachers and thus in teacher–student interactions.