Discussion with a Christian about the Crucifixion
Question

Why is it so difficult for the Muslims to believe that Christ was crucified to erase our sins? Why do they reject the idea of the Crucifixion altogether?.
Praise be to Allah.There is nothing strange about the Muslims rejecting this idea, because the Quran in which they believe and accept what it tells them definitively states that that did not happen, as Allah says (interpretation of the meaning): 
“And because of their saying (in boast), ‘We killed Messiah ‘Eesa (Jesus), son of Maryam (Mary), the Messenger of Allah,’ — but they killed him not, nor crucified him, but it appeared so to them the resemblance of ‘Eesa (Jesus) was put over another man (and they killed that man)], and those who differ therein are full of doubts. They have no (certain) knowledge, they follow nothing but conjecture. For surely; they killed him not [i.e. ‘Eesa (Jesus), son of Maryam (Mary)]”
[al-Nisa’ 4:157]
Rather the problem rests with the Christians for whom the doctrine of the crucifixion and redemption has become a central issue, so much so that the cross is the symbol of their religion. 
It is strange that they differ concerning the form of this cross which indicates their confusion about this fabrication. 
There are differences between their Gospels and their historians regarding everything that has to do with the story of the crucifixion. 
They differ concerning the timing of the Last Supper, which according to them was one of the events in the lead-up to the crucifixion. They differ concerning the traitor who led (the Romans) to Christ – did that happen at least one day before the Last Supper, as narrated by Luke, or during it, after Christ gave him the piece of bread, as narrated by John? 
Was Christ the one who carried his cross, as John says, as was customary with one who was going to be crucified, according to Nottingham, or was it Simon of Cyrene, as the other three Gospels state?
 They say that two thieves were crucified alongside Christ, one on his right and one on his left, so what was the attitude of these two towards the Messiah who was being crucified, as they claim? 
Did the thieves scorn him for being crucified, and say that his Lord had abandoned him and left him to his enemies? Or did only one of them scorn him, and did the other rebuke the one who scorned him?  
At what hour did this crucifixion take place – was it in the third hour, as Mark says, or in the sixth as John says? 
What happened after the so-called crucifixion?  
Mark says that the veil of the Temple was torn from top to bottom. Matthew adds that the earth shook and rocks crumbled, and many of the saints rose from their graves and entered the holy city, appearing to many. Luke says that the sun turned dark, and the veil of the Temple was torn in the middle, and when the centurion saw what had happened, he glorified God and said, “Truly this man was righteous.” 
But John does not know anything about all that! 
These are not the only weak elements and indications of falseness in the story of the crucifixion, as narrated in the gospels. Rather the one who studies the details of the gospel narratives of this story will, with the least effort, notice the great differences in the details of this story, which are such that it is impossible to believe it all or even any part of it! 
How desperate are the failed attempts to fill this gap and conceal the faults of this distorted book. Allah indeed spoke the truth when He said in His Book which He has preserved (interpretation of the meaning): 
“Do they not then consider the Quran carefully? Had it been from other than Allah, they would surely, have found therein many a contradiction” [Al-Nisa’ 4:82]
Apart from the fact that the gospel accounts are not sound, and their authors themselves admit that they were not revealed to the Messiah in this form, nor were they even written during his lifetime, none of the witnesses were present at the events to which they testify, as Mark says:  
“Then everyone deserted him and fled.”
Mark 14:50 – New International Version (NIV)
Because these events were not witnessed by anyone who narrated them, there is a great deal of room for imagination and poetic licence. 
We will complete our discussion of the fable of the crucifixion of Christ (peace be upon him) by looking at what the Gospels say about the Messiah’s prediction that he would be saved from death: 
On one occasion the Pharisees and chief priests sent the guards to arrest him and he said to them: 
“I am with you for only a short time, and then I go to the one who sent me. You will look for me, but you will not find me, and where I am, you cannot come.”
John 7:33-34 – NIV
Elsewhere he says: 
“Once more Jesus said to them, ‘I am going away, and you will look for me, and you will die in your sin. Where I go, you cannot come.’
This made the Jews ask, ‘Will he kill himself? Is that why he says, “Where I go, you cannot come”?’
But he continued, ‘You are from below; I am from above. You are of this world; I am not of this world.
I told you that you would die in your sins; if you do not believe that I am the one I claim to be, you will indeed die in your sins.’
‘Who are you?’ they asked.
‘Just what I have been claiming all along,’ Jesus replied. ‘I have much to say in judgment of you. But he who sent me is reliable, and what I have heard from him I tell the world.’
They did not understand that he was telling them about his Father.
So Jesus said, ‘When you have lifted up the Son of Man, then you will know that I am the one I claim to be and that I do nothing on my own but speak just what the Father has taught me.
The one who sent me is with me; he has not left me alone, for I always do what pleases him.’”
John 8:21-29 – NIV
Then at the end he tells them again: 
“For I tell you, you will not see me again until you say, 'Blessed is he who comes in the name of the Lord.'”
Matthew 23:39 – NIV, also Luke 13:35
The Messiah, as these texts and others show, was certain that God would never hand him over to his enemies, and would never forsake him. 
“But a time is coming, and has come, when you will be scattered, each to his own home. You will leave me all alone. Yet I am not alone, for my Father is with me.
I have told you these things, so that in me you may have peace. In this world you will have trouble. But take heart! I have overcome the world.”
John 16:32-33
Because of that the passers by, and indeed everyone who attended the so-called crucifixion, mocked the Messiah, as the writer of this Gospel says (although that could not have been true): 
“Those who passed by hurled insults at him, shaking their heads
and saying, ‘You who are going to destroy the temple and build it in three days, save yourself! Come down from the cross, if you are the Son of God!’
In the same way the chief priests, the teachers of the law and the elders mocked him.
‘He saved others,’ they said, ‘but he can't save himself! He's the King of Israel! Let him come down now from the cross, and we will believe in him.
He trusts in God. Let God rescue him now if he wants him, for he said, “I am the Son of God.”’
In the same way the robbers who were crucified with him also heaped insults on him.”
Matthew 27:39-44 – NIV
But it seems that Jesus’ certainty that God was with him began to waver, according to the distorted Gospel narrative, (although that could not have been true): 
“Then Jesus went with his disciples to a place called Gethsemane, and he said to them, ‘Sit here while I go over there and pray.’
He took Peter and the two sons of Zebedee along with him, and he began to be sorrowful and troubled.
Then he said to them, ‘My soul is overwhelmed with sorrow to the point of death. Stay here and keep watch with me.’
Going a little farther, he fell with his face to the ground and prayed, ‘My Father, if it is possible, may this cup be taken from me. Yet not as I will, but as you will.’
Then he returned to his disciples and found them sleeping.
 
…
He went away a second time and prayed, ‘My Father, if it is not possible for this cup to be taken away unless I drink it, may your will be done.’
When he came back, he again found them sleeping, …
So he left them and went away once more and prayed the third time, sayingthe same thing.
Then he returned to the disciples and said to them, ‘Are you still sleeping and resting? Look, the hour is near, and the Son of Man is betrayed into the hands of sinners’”
Matthew 26:36-45 – NIV
Luke describes the scene and says: 
“And being in anguish, he prayed more earnestly, and his sweat was like drops of blood falling to the ground.
When he rose from prayer and went back to the disciples, he found them asleep, exhausted from sorrow.
‘Why are you sleeping?’ he asked them. ‘Get up and pray so that you will not fall into temptation.’”
Luke 22:44-46 – NIV
Because of this mockery of the message of Christ – according to their claims – and because Christ thought that God was with him and would never forsake him, then it follows that the writer who fabricated this dramatic scene would end it with a vision of the despair of the Messiah and his feelings of being abandoned by God – exalted be Allah far above what the wrongdoers say. The fabricator says: 
“From the sixth hour until the ninth hour darkness came over all the land.
About the ninth hour Jesus cried out in a loud voice, ‘Eloi, Eloi, lama sabachthani?’--which means, "My God, my God, why have you forsaken me?"
Matthew 27:38-47 – NIV
See also Mark 15:34
If we understand what this story means when subjected to criticism, the same will apply to the doctrine of redemption and sacrifice that is based on it. 
With regard to the Christian doctrine of salvation, see also question no. 6 
And Allah is the Source of strength and the guide to the Straight Path, and there is no Lord but He.