From Gerd Luedemann, The Resurrection of Jesus: History, Experience, Theology (Minneapolis, MN: Fortress, Press, 1994), 99-100.
To further an understanding of Peter’s “mourning” and “vision”, reference should be made in this connection to investigations at Harvard into cases of mourning and the painful loss associated with them. The researcher followed forty-three widows and nineteen widowers through the process of mourning and interviewed them at intervals of three weeks, eight weeks and thirteen months after the death of the partner. The aim of the work was to investigate what made it possible to work through mourning. Among other things three factors were mentioned which prevented mourning: 1. a sudden death; 2. an ambivalent attitude to the dead person associated with guilt feelings; and 3. a dependent relationship.
Applying this finding to the situation of Peter and the disciples, we should note that all three factors which make mourning difficult apply to them: 1. the crucifixion of Jesus happened unexpectedly and suddenly; 2. the relationship of the disciples to Jesus was marked by ambivalence and guilt feelings: Judas betrayed Jesus and then committed suicide; Peter denied Jesus and wept bitterly; 3. a dependent relationship of the disciples on Jesus can be seen in the fact that most had left their work and homes to be with him. The dependence was perhaps further intensified by the fact that the followers of Jesus represented a small religious group which had detached itself from its original social structures and thus had formally parted company with the outside world. Jesus was one and all to them. (Granted, these are conjectures, but they may have a historical foundation.)
Conclusion: the mourning hindered by the three factors mentioned was enormously helped in the case of Peter by a vision, indeed concentrated in a moment of epiphany. The mourning first led to a deeper understanding of Jesus, and this in turn helped towards a new understanding of the situation of mourning. Recollections of who Jesus was led to the recognition of who Jesus is. Seeing Jesus here included a whole chain of (potential!) theological conclusions.