The goal of this article is to consider the literary-theological function of the hardening of the disciples' hearts in Mark 8:14-21. The disciples are remarkably characterised by faithlessness, which is associated with hardness of their hearts. Although Mark uses the same language, 'hardness of heart', at different points in his Gospel to describe both Jesus' opponents and the disciples, he nevertheless retains a distinction between the two groups. With regard to the opponents' unbelief, the language means a divine judgement for their unbelieving rejection (cf. Mark 3:5-6). By contrast, when the language is used in relation to the disciples, it warns them (or the Markan readers) to beware of falling into the opponents' unbelieving attitudes (6:52; 8:17-18).