The focus of Mark 15:21–39 is the crucifixion of Jesus, specifically, the theological implications of Jesus’ death. On the cross we see the cost of sin. In the death of Jesus, we see the penalty that sin requires: physical suffering, shame and humiliation, and spiritual suffering.
21 And they compelled a passerby, Simon of Cyrene, who was coming in from the country, the father of Alexander and Rufus, to carry his cross.