In Mark's Gospel, when he speaks of the resurrection of Christ, he repeatedly mentions the unbelief of the apostles. He records the devotion of the women, but he also implies their unbelief too.
The Lord made a promise to them, and that promise would apply if – here was the condition – they kept up their belief. Now of course they would keep up their belief; they were truly saved men. They would not be lost. They would be faithful to him for the rest of their lives, most of them probably dying in martyrdom, but still their belief from day to day, week to week, month to month, may waver, and it’s so with us. We've come to Christ, let us say; we've repented of sin; we’ve trusted in him and his shed blood on Calvary for sinners. We’ve experienced illumination and new life in conversion. We believe, and yet we may be only half believing. We may be believing the facts of redemption, and believing that we are saved. We may be believing in Christian duties, believing our worship, believing many of our prayers, and yet other things have somehow slipped from our minds, and it is as though we hardly believe them at all. So when trouble strikes, or grief or difficulties come, or strong temptations, we flounder and we are floored. The old nature begins to speak, and perhaps we become irritable and bad tempered, and show all kinds of wrong reactions, because we do not believe. Curiously, even as believers, even as seasoned believers, we do not believe in the continuing presence of God with us and his continuing power – at least we don’t act as though we do. We do not believe in his providence. We do not continue to believe, as we normally claim we do, that all things work together for good to them that love God. It seems to have gone and we panic. So we understand what is going on with the disciples when we look at them in the final chapter of Mark as unbelieving believers, because they did not believe the resurrection. Even having seen the risen Christ, some wavered.