And they say, “The Most Merciful has taken [for Himself] a son.” You have done an atrocious thing. The heavens almost rupture therefrom and the earth splits open and the mountains collapse in devastation. That they attribute to the Most Merciful a son. And it is not appropriate for the Most Merciful that He should take a son. There is no one in the heavens and earth but that he comes to the Most Merciful as a servant. 
Despite this, this statement is mentioned upon the tongues of different polytheistic religions in the Qur’ān many times. It is thus with a cautious, discomforted sadness that a monotheist writes about beliefs which are antithetical to glorifying and magnifying the Almighty, who is in no need of anything, in order to fairly discuss elements where we disagree, to correct their mistake and make the truth concerning this manifest.
Conditioning Sacrifice for Forgiveness Generally
It is accepted in all faiths and in all doctrines (even by one’s natural disposition) that every person is responsible for the bearing of his own burden. One should not bear the sins reaped by others that are beyond one’s choice and control while one should accept liability for his or her own. Conditioning ‘sacrifice’ of another human being as the only means for forgiveness is problematic from a number of angles.
Firstly, it does not suit the justice of God that He decrees that all of humanity’s sins fall on one individual who was among their most righteous, Jesus the son of Mary. Allāh says in the Qur’ān:
“Whoever goes right, then he goes right only for the benefit of his own self. And whoever goes astray, then he goes astray to his own loss. No one laden with burdens can bear another’s burden. And We never punish until We have sent a Messenger.” 
And it is mentioned in the Bible, in Ezekiel 18:20:
“The son shall not bear the iniquity of the father; neither shall the father bear the iniquity of the son: the righteousness of the righteous shall be upon him, and the wickedness of the wicked shall be upon him.”
With the knowledge of future sins already cleansed, belief that Jesus bore the ‘original sin’ or all our sins can further incentivise sinners and avert people from engaging in good deeds if these good deeds are challenging to perform. Some argue that a sinner has in fact ‘not accepted’ the death of Christ for our sins but, by the assertion of the Bible, we all hold in ourselves the tendency to sin. So what sin, if performed, constitutes one ‘not accepting’ the death of Christ: any sin or a particular sin? If we say ‘any sin’, then no one can possibly accept this doctrine because we all sin to some degree. If we say ‘a particular sin’, then it cannot be said that Jesus died for all our sins without qualifying this statement. Many can and indeed have used this concept to justify their evil. On top of this, equating the righteous to the transgressor is also a way of attributing injustice to God.
“Not equal are the dwellers of the Fire and the dwellers of Paradise. It is the dwellers of Paradise that will be successful.” 
Selective Justice is not Justice
Many of today’s Christians hold that if God is to welcome us into His presence in Heaven but also preserve His justice and holiness, something more than animal sacrifice, good action and sincere repentance is required as the latter is man’s action; as valuable to God as “dirty rags” stained by menstrual blood (according to Isaiah 64:6). This indeed implies that none before Jesus attained God’s full mercy as Jesus had not died then. Obviously, this is an impossible preservation of His justice and holiness as these preceding generations died before having the ability to acknowledge the ‘atonement’ that many Christians require acknowledgement of. Only if Jesus was sacrificed for our sins before the existence of humanity would justice be better served for the succeeding generations, but then there would be no sins for the sacrifice to be necessary!
Christians may assert that animal sacrifices practiced by the previous generations, mentioned extensively in the Torah, are a ‘foreshadowing’ of the sacrifice of Christ, but they are hardly accepted by God. This is built on the premise, however, of believing Jesus died for our sins, where the former was stated to fit the latter presumption. Any figurative interpretation is subjective and depends on an underlying narrative in this way. In turn, any subjective interpretation, that forms one’s creed needs clear, objective and unambiguous evidence to support it.
Moreover, believing that Jesus’ sacrifice better serves God’s justice and holiness is also problematic. This implies that God’s holiness can be served and his right due can be fulfilled, which is impossible of an infinitely great deity.
The belief that Jesus was the divine deity, God and Man, or God manifesting into man, attributes death (a creation of God) to God and claims that a creation (death) overtook God. If he were the Son of God (and His refuge is sought from this), it asserts that the Son (a lesser entity) is enough to fulfil the due right of the grander, greatest entity, when Jesus is claimed to have said: “The ‘Father’  is greater than I” (John 14:28).
Further to this:
1) Actions do not increase Allāh in His dominion nor is He in need of them. But He accepts them because He is the All-Merciful. To claim that the death of Christ is the only thing that God accepts for forgiveness is to claim that God has no mercy at all.
2) Alleging that whatever previous generations did (including Prophets) was not enough for them to earn ‘complete’ forgiveness, implies that God’s mercy is bound to a time or era and that this characteristic is not timeless which contradicts the monotheistic precinct that His characteristics are timeless.
3) Any newborn that dies before the advent of Christ is doomed to punishment having inherited sin, whilst any christened newborn thereafter is saved, thus assigning partiality to God’s forgiveness which is not befitting of One whose forgiveness and mercy extends to all His creatures:
“…and My Mercy embraces all things…” 
4) In the absence of the above, if we affirm that God’s mercy is impartial, which is the only belief suitable for one convinced that God is the creator and cherisher of all, then the preceding generations must have enjoyed the same opportunity to make repentance as we do today (hence the many stories in the Bible affirming the forgiveness of individuals in response to repentance or other defined action). Saying this, whatever their law stipulated for attaining forgiveness must have been sufficient for them especially as these actions were legislated by the Prophets of their times who received revelation from God directly.
So, if sacrificing an animal earned one forgiveness in the past by the admission of the Bible, what of a person who humbles himself before God, weeping in regret and resolving to abstain from the sin that he fell into? He is for sure worthy, or worthier of forgiveness.
“Despair not of the Mercy of Allāh: verily, Allāh forgives all sins. Truly He is Oft-Forgiving, Most Merciful. “And turn in repentance and in obedience with true Faith (Islamic Monotheism) to your Lord and submit to Him (in Islām)…” 
Leading on from this point, God, Allāh, created man out of His mercy, not as a result of a person’s deeds. Our eyesight, for instance, is far more valuable than anything we can offer in return for it. With this in mind, would God lend man this utter and complete mercy, free of charge, during creation without us asking for it, whilst prohibiting this individual from His mercy if it is invoked and sincerely requested? Surely not. Would the Most Merciful not endow man with the tools required to attain His mercy directly, and to all unreservedly?
Incarnation of God into Man? The Need for Clarity of Doctrine
The believed incarnation of God into Jesus is inconsistent with even the current Bibles’ own repeated and oft-emphasised separation of Jesus and God. Not a single time did Jesus make it unequivocally explicit that ‘I am God,’ or ‘I am the incarnation of God’ or even ‘worship me’ in the entire text of the Bible. Rather Jesus himself is claimed to have emphasised that he did not speak on his own authority (John 14:10) nor did he do anything by his own authority, but he spoke only what the “Father” had taught him (John 8:28). If Jesus was God, or God is a trinity, it begs the question of why he did not say this in response to the man who addressed him with the distinctly Islamic declaration:
“Well said, teacher, you are right in saying that God is one and there is no other but him…” (Mark 12:32)
This goes a long way in clarifying who Jesus actually was. A righteous Prophet and servant of God falls more in tandem with the explicit text of the Bible, the Qur’ān and the Torah, whilst the understanding adopted by Christians in latter eras conflicts with each of these. Interestingly, the scholar of Christian scripture, Professor Reza Aslan, who studied the historical Jesus of Nazareth for over two decades, highlights the source of the divergent strands of Christianity where Jesus was raised to divine status, and salvation only came through belief, rather than through following the previous laws. He describes the early historical Jesus as a revolutionary teacher who sought to bring an end to Roman occupation of Jerusalem and was crucified by the Romans for attempted treason. After the Jewish rebellion, in which thousands of Jews were slaughtered by Roman legions only decades after Jesus’ mission, the Roman Hellenised Christians—lead by Saul, later known as Saint Paul— sought to pacify and secularise his teachings. They reinterpreted and rewrote them to suggest he was not sent to save the Jews from Roman occupation but, rather, to offer a metaphorical salvation which did not involve any opposition to Rome.
So, if the divinity of Christ is the backbone of western Christianity, with all of the biblical uncertainty surrounding this topic, it becomes problematic to further derive from this problematic belief aspects relating to the atonement and so on.
If one were to ask, ‘What is the fundamental message of Islām?’, almost every Muslim would respond with Monotheism – singling out Allāh as the Lord, the worthy deity of worship and the individual possessor of His own attributes. Almost everything in the Qur’ān points to this, more often than not explicitly: “Say He is Allāh, the One”, “Verily your Lord is One Lord, there is no God but He,” “There is nothing like unto Him,” “Allāh, there is no deity worthy of worship but He, the Hayy (the Living) the Qayūm (the One Who Sustains),” and so on and so forth. Why is such clarity surrounding the divinity of Christ not espoused by the Bible? Especially if it is now suggested as a pre-condition for salvation from eternal damnation.
And all of the above is said whilst assuming Jesus was killed which is not a belief Muslims hold in the first place. Even if by assuming he was killed, we fail to reach the conclusions drawn by modern-day Christians, what then if he did not die at all, but was raised up to the Heavens to return at an appointed time?
“I am ascending to my Father  and your Father, to my God and your God” (John 20:17).
“And he (‘Īsā (Jesus), son of Maryam (Mary)) shall be a known sign for [the coming of] the Hour (Day of Resurrection) [i.e. ‘Īsā’s (Jesus) descent on the earth].” 
The unambiguous reality is as Allāh, the Most High, states:
“…but they killed him not, nor crucified him, but it appeared so to them, 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. But Allāh raised him up unto Himself…”
Putting all the uncertainty aside, it behoves the one sincerely seeking nearness to his or her creator to make those things which are crystal clear their basis for approach, and interpret the ambiguous or subjective in its light. In their heart of hearts, one who believes in the justice of God will agree that God would not expect His servants to ignore His clear and unambiguous statements telling them to approach Him alone and directly, and instead follow an uncertain route to Him. The final Prophet Muhammad (peace and blessings be upon him and all the Prophets and Messengers) sent to the Christian kings of his time what God says in the Qur’ān:
Say, “O People of the Scripture, come to a word that is agreeable between us and you – that we will not worship except Allāh and not associate anything with Him and not take one another as lords instead of Allāh.” But if they turn away, then say, “Bear witness that we are Muslims [submitting to Him].”
Notes: http://christianity.about.com/od/holidaytips/qt/whatiseaster.htm  Al-Qur’ān 19:88-93  Al-Qur’ān 17:15  Al-Qur’ān 59:20  Shaykh al-Islām Ibn Taymiya wrote that the original word ‘Father’ in some ancient languages has been suggested to mean similar to the Arabic word ‘Rabb’ (Lord; Master). This may account for the current translation of ‘Father’ to refer to God in the Christian scriptures. However, since Allah has not used this term for himself explicitly, and it carries potentially negative connotations, it is impermissible for Muslims to refer to Him – subhānahū wa ta’ālā – using this name.  Al-Qur’ān 7:156  Al-Qur’ān 39:53-54  Zealot: The Life and Times of Jesus of Nazareth, by Reza Aslan. Published by The Westbourne Press (3 Mar. 2014)  Al-Qur’ān 43:61  Al-Qur’ān 4:157-158  Al-Qur’ān 3:64