New Defender's Study Bible Notes
15:21 Simon a Cyrenian. This incident is mentioned in all three synoptic gospels. Cyrene was a city in North Africa. Simon evidently became a Christian; otherwise it is doubtful that the writers would have known his name, and especially the names of his sons.
15:21 Alexander and Rufus. Matthew and Luke also mention Simon, but only Mark mentions Alexander and Rufus, presumably because he knew the family. Rufus probably became a Christian, for there is a Rufus mentioned by Paul in Romans 16:13.