bismillah
assalamu alaikum,
Knowing well that the Beloved Isa ibn Maryam (as) was not killed nor crucified. The Nasara are lead to believe in two doctrines not formulated till years after Isa ibn Maryam (as) departure.
I will give you the Christian doctrine specifically:
http://en.wikipedia.org/wiki/Salvation
"...Salvation is arguably one of the most important Christian spiritual concepts, perhaps second only to the divinity of Jesus Christ.
Among many Christians, the primary goal of religion is to attain salvation. Others maintain that the primary goal of Christians is to do the will of God, or that the two are equivalent. In many traditions, attaining salvation is synonymous with going to heaven after death, while most also emphasize that salvation represents a changed life while on Earth as well. Many elements of Christian theology explain why salvation is needed and how to attain it.The idea of salvation rests upon there being some sort of unsaved sinful state from which the individual (or mankind) is to be redeemed by a Saviour. This Saviour is Jesus Christ..."
In a nutshell, humanity from the time when Adaam and Huwa disobeyed Allaah (swt) and listened to the whispers of the shaitan, humanity was in dire need of a "perfect sacrifice" in order to liberate us from the inevitable doom
of the hellfire, in which with the belief in Isa (as) as lord and savior, where are going.
Regarding his supposed death there are Christians who believe that Isa (as) was merely a man i.e Unitarian Christianity, thos who believe he was God and man (Trinitarianism Christianity). Others did not believe he was crucified, and other believe the death was a spiritaul demise and not a physical one.
Christian NT textual proof:
"For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life." - John 3:16
Hebrews 1:5:
"For unto which of the angels said he at any time, Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son?"
John 10:23
"And Jesus walked in the temple in Solomon's porch. Then came the Jews round about him, and said unto him, How long dost thou make us to doubt? If thou be the Christ, tell us plainly. Jesus answered them, I told you, and ye believed not: the works that I do in my Father's name, they bear witness of me. But ye believe not, because ye are not of my sheep, as I said unto you. My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand. I and my Father are one."
John 8:56-59
"before Abraham was born, I am" (this verse is used to prove the 'supposed divinity of Isa (as).
"And if a man have committed a sin worthy of death, and he be to be put to death, and thou hang him on a tree: His body shall not remain all night upon the tree, but thou shalt in any wise bury him that day; (for he that is hanged is accursed of God;) that thy land be not defiled, which the LORD thy God giveth thee for an inheritance."
Deuteronomy 21:22-23 (this verse is said by the yahud to prove he was their Masih. But the Nasara say that Isa (as) became a curse for us, thus they are justified.
The point is this is what they sincerely believe and this what they preach to the nations. I love my brothers in humanity, but it is a necessity, to study the early church in order to understand more comprehensively the circumstances that shaped the Christianity we have today. Wa Allaahu alim.