Countless articles and monographs have been published which have paid attention to the various themes in Mark’s Gospel. However, to date there is no publication that attempts to understand the theme ‘unbelief’ in Mark’s Gospel. This study attempts to investigate the literary-theological functions of unbelief as a theme in Mark. This approach is based on the exegetical perspective of several passages (2:1-12; 3:1-6; 4:35-41; 6:1-6; 8:14-21; 9:14-29; 11:27-33; 15:27-32) in which the theme of unbelief appears. Given this approach, it becomes clear that, although at different points of his Gospel, Mark applies the term ‘unbelief’ used to depict Jesus’ opponents to the disciples as well, he distinguishes between the two groups. This applies also to his description of the unbelief of both the groups. The opponents’ unbelief does not mean primarily a lack of insight, but rather a conscious refusal to believe Jesus’ claims and demands. Throughout Mark’s Gospel the opponents, due to their hardened hearts, they are ever seeing and hearing but never understanding (3:5; 4:12). Similar to 1QS 3:18-21, in Mark 4:15 the fundamental source of the opponents’ rejection is the result of Satan-inspired opposition (1QS 3:18-21; Mark 4:15) Since Satan prevents the opponents from listening to Jesus’ message, they do not understand it and reject him and eventually bring about his death (3:6). Thus, to the opponents, Jesus’ salient teachings and miracles have only produced rejecting questions, rather than belief (cf. 3:6, 21, 22-29, 30-31; 6:1-6). Therefore, their obstinate rejection necessarily excludes them from obtaining forgiveness of God. However, if they repent of their sins and accept His prophetic message, they will gain God’s forgiveness as a benefit in the Kingdom (cf. 12:34). On the other hand, the disciples’ unbelief, unlike the opponents’ unbelief, does not consist in the wilful rejection of Jesus. But, the disciples’ unbelief implies their failure to believe in Jesus’ identity (4:35-41; 6:45-52; 8:14-21; 9:14-29 etc.) shown in his teachings and actions. The disciples are in danger of being the outsiders whose hearts were hardened, having eyes but not seeing, and ears but not hearing (8:17-18; cf. 3:5; 4:11-12). Although the hardness of heart, which is associated with Jesus’ opponents, is also attributed to the disciples, the use of this term is not meant to suggest that the disciples have sided with the opponent of Jesus. Rather, it is indicative of an attempt by Jesus, by bombarding them with a series of rhetorical questions, to warn his disciples who are in mortal danger of succumbing to the same unbelief that has afflicted the opponents. Thus, unlike a direct statement regarding the opponents’ hardness of heart in 3:5 and 4:12, the disciples’ hardness of heart is obliquely described through rhetorical questions in 8:17-18. The theological perspective that is presented in the theme of unbelief in Mark is that an attempt is made to emphasize faith, which stands literally in contrast to unbelief. Through speech acts concerning unbelief, Mark emphasizes the fact that those who are the true disciples must follow Jesus with faith under difficult circumstance. For this purpose, he uses specific individuals as examples (the woman with a haemorrhage, Jairus, the paralytic, and the Syrophoenician woman) who respond to Jesus’ demand with faith and spiritual insight, in order to move Christians toward faith or trust in him along with a perceptiveness of his identity.