Charles Spurgeon Commentary Mark 15:21

"And they compel one passing by, Simon of Cyrene, coming from the country, the father of Alexander and Rufus, to go [with them], that he might bear his cross." — Mark 15:21 (ASV)

And they compel one Simon a Cyrenian, who passed by, coming out of the country, the father of Alexander and Rufus, to bear his cross.

Christ did not bear it himself; the soldiers saw that he was faint and weary, so they laid the cross, or at least one end of it, on Simon's shoulders.