Why was it necessary?
What happened in the “forcing” of Simon was common Roman custom and quite legal.
Jesus refers to it in
Matthew 5:41 (NIV)
41 If someone forces you to go one mile, go with him two miles.
But why was it necessary?
1 The soldiers had pity on Jesus? NO
They throughout the crucifixion mock him and indulge in every refinement of cruelty. There is no compassion here for the one who has such compassion on them.
Read Mark 15:16-20
Mark 15:35-36 (NIV)
35 When some of those standing near heard this, they said, “Listen, he’s calling Elijah.”
36 One man ran, filled a sponge with wine vinegar, put it on a stick, and offered it to Jesus to drink. “Now leave him alone. Let’s see if Elijah comes to take him down,” he said.
2 It reflects upon the real sufferings of Jesus? YES
It lets us see how truly human our Master was. He had been all nigh-time in the garden, sweating as it were great drops of blood in his anguish: he had been before the Sanhedrim, he had been before Pilate, then before Herod, then before Pilate again; he had endured scourging; he had been mocked by the soldiery; and it would have been a great wonder if the human frame had not shown some sign of exhaustion.
There was an early heresy that denied the real sufferings of Christ
Therefore (because he was Mind) he did not suffer, but a certain Simon of Cyrene was impressed to carry his cross for him, and because of ignorance and error he was crucified, transfigured by him so that he might be thought to be Jesus: and Jesus himself assumed the form of Simon and, standing by, laughed at them
3 It is a small testimony to the purity of Jesus. YES
One thing is sure: the bearing of the cross was a mark of guilt; and Jesus was not guilty (see Mark 15:20–21 and Rom. 16:13).
4 It served the sovereign purposes of God. YES
a To show mercy to one man.
b Through that one man a whole family.
c To demonstrate discipleship to all Christians.
Who was this Simon of Cyrene?
Cyrene city in Cyrenaica (modern Libya) which had a thriving Jewish community of settlers from Egypt from Ptolemaic times (late fourth century b.c.). Cyrenian Christians were prominent in Antioch (Acts 11:20; 13:1)
He may possibly have been black if he is to be identified with Simon Niger of (Acts 13:1–3).
Surnamed Niger, i.e., “black,” perhaps from his dark complexion, a teacher of some distinction in the church of Antioch (Acts 13:1–3). It has been supposed that this was the Simon of Cyrene who bore Christ’s cross. Note the number of nationalities represented in the church at Antioch. 
He was almost certainly converted either at this moment or later on.
To us it seems altogether likely that the conversion of Simon the Cyrenian dated from that memorable day when “passing [casually] by, as he came from the country” (Mk 15:21), “they compelled him to bear the” Saviour’s cross. Sweet compulsion, if what he thus beheld issued in his voluntarily taking up his own cross! 
We have good reason to believe that Simon was converted because of this encounter with Jesus. Mark identified him as “the father of Alexander and Rufus” (Mark 15:21), two men that Mark assumed his Roman readers would know. A Christian named Rufus was greeted by Paul in Romans 16:13, and it is possible that he was the son of Simon of Cyrene. Apparently Simon and his two sons became well-known Christians who were held in honor in the church.
The link with Rome is compelling, Mark wrote at Rome Paul is writing to Rome.
Romans 16:13 (NIV)
13 Greet Rufus, chosen in the Lord, and his mother, who has been a mother to me, too.
We can see the rich fruits of conversion.
A man is saved.
His wife becomes like a mother to the Apostle Paul.
His son is described as “choice” among the elect.
Chosen in the Lord. Elected to salvation. Some translations render “chosen” as “choice,” which indicates he was widely known as an extraordinary believer because of his great love and service. his mother and mine. Rufus was not Paul’s natural brother. Rather, Rufus’ mother, the wife of Simon of Cyrene, at some time had cared for Paul during his ministry travels.
!! What lessons are there for us?
There is a discernable theme in Mark where he sets minor characters over against disciples to show the duties they often neglect.
In light of the desertion of the disciples, Mark once again in the passion narrative contrasts minor characters with the disciples. Minor characters fulfill certain duties that the disciples should accomplish but do not, because they abandon their responsibility to follow Jesus. Mark expresses the action of Simon of Cyrene who carries Jesus’ cross (15:21 ), with words of taking up the cross (8:34 ). Thus Simon of Cyrene serves Jesus in a way that is appropriate for a follower.
All that will approve themselves disciples indeed, must follow Christ, bearing his cross (ch. 16:24), bearing his reproach, Heb. 13:13. We must know the fellowship of his sufferings for us, and patiently submit to all the sufferings for him we are called out to; for those only shall reign with him, that suffer with him; shall sit with him in his kingdom, that drink of his cup, and are baptized with his baptism. Matthew Henry
In a sermon on Christ as our husband George Whitefield
Persons that are married, take one another for better or for worse, for richer or for poorer, to love and to cherish each other in sickness and in health. And if we are married to Jesus Christ, we shall be willing to bear his cross, as well as to wear his crown. “If any man will come after me, let him deny himself, take up his cross, and follow me.” Neither will they be compelled to do this, as Simon of Cyrene was, but they will be volunteers in his service; they will cry out, Crown him, crown him ,when others are crying out, “Crucify him, crucify him.” They will never leave or forsake him, but willingly follow the Captain of their salvation, though it be through a sea of blood. 
In 1882 Charles Spurgeon preached on similar challenging lines
His name was Simon: and where was that other Simon? What a silent, but strong rebuke this would he to him. Simon Peter, Simon son of Jonas, where wast thou? Another Simon has taken thy place. Sometimes time Lord’s servants are backward where they are expected to be forward, and he finds other servitors for the time. If this has ever happened to us it ought gently to rebuke us as long as we live. Brothers and sisters, keep your places, and let not another Simon occupy your room. It is of Judas that it is said, “his bishopric shall another take;” but a true disciple will retain his office. Remember that word of our Lord, “Hold that fast which thou hast, that no man take thy crown.” Simon Peter lost a crown here, and another head wore it .
 Charles Spurgeon preached October 8th 1882 No. 1683
The Journal of the Evangelical Theological Society. 1998 Garland, TX:
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Easton, M. (1996, c1897). Easton's Bible dictionary.
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 Discipleship and Minor Characters in Mark’s Gospel Joel F. Williams is Assistant Professor of Bible, Columbia International University, Columbia, South Carolina. Bibliotheca Sacra.
Henry, M. (1996, c1991). Matthew Henry's commentary on the whole Bible : Complete and unabridged in one volume (Mt 27:26). Peabody: Hendrickson.
Whitefield, G. (1999). Selected Sermons of George Whitefield. Oak Harbor, WA:
 Charles Spurgeon preached October 8th 1882 No. 1683