Islam seems so Old Testament sometimes, that it's easy to fall into the trap of thinking it is essentially a 7th-century revival of the religion of Moses and Joshua, a throwback to pre-Christian (and pre-Talmudic) times. In fact, though, some of the central concepts of Islam are distinctively
Christian rather than Mosaic. Here I want to look at some of the main Old and New Testament currents in the thought of the Quran. I refer to books of scripture rather than to religions because both Christianity and Islam have long histories and many sects, embracing a variety of ideas and emphases. I will also be looking only at central concepts, not surface-level details like circumcision and not eating pork. As a final caveat, I should note that I haven't actually read the Quran in nearly 15 years and will be going by memory here.
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Old Testament ideas in the Quran
1. Monoloatry. The exclusive worship of one God is the primary religious duty. Idolatry and the worship of other gods are the primary sins. This is absolutely central to both the Old Testament and the Quran but is not mentioned at all in any of the Gospels. It became an issue again when Paul took Christianity to the Greeks and Romans, but Paul clearly got it from the Old Testament, not from anything that Jesus taught.
2. Theocracy. God's law is to be enforced, and those who offend him are to be punished, typically with either exile or death. Living under Sharia would be very similar to living under the Mosaic theocracy. Jesus, though, even though as the Messiah he was expected to reestablish a theocracy, explicitly refused to do so. Later Christian history would feature such things as the Inquisition, of course, but this does not come from the New Testament.
3. Jihad. God's people are literally at war with those who serve other gods and should try to exterminate them and their religion. This is central to the Torah and the Deuteronomistic History, and of course to the Quran as well. The jihad concept is largely absent from the New Testament, except as an occasional metaphor, and Jesus sometimes taught the direct opposite: resist not evil, turn the other cheek, etc.
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New Testament ideas in the Quran
1. Belief. Of course everyone with a religious message hopes that that message will be believed, but the New Testament and Quran stand out for their extreme emphasis on the moral duty to believe in God and his messengers. The Gospel of John alone uses the word believe twice as many times as the entire Old Testament and says "This is the work of God, that ye believe on him whom he hath sent." The Quran constantly inveighs against "those who take God's messengers for liars." The Old Testament, for all its holy wars, contains not a single reference to "unbelievers" or "infidels"; rather, the enemy are the uncircumcised.
2. Heaven and hell. There is no real afterlife in the Old Testament -- only sheol, which seems sometimes to be something like the Homeric afterlife (an undifferentiated realm of half-conscious shades) and sometimes just a name for "the grave." God's rewards and punishments were expected to come in this life -- either one's own life or that of his descendants. Heaven and hell were introduced by Jesus and are a major theme of the Quran as well. (The Quranic paradise has often been characterized as crudely materialistic -- "gardens beneath which rivers flow" -- but I'm not sure that is fair. Christians have used similar metaphors to describe Heaven, and there is no obvious reason to assume Muhammad was being literal.)
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The central synthesis
The fundamental Islamic credo -- "There is no god but Allah, and Muhammad is his prophet" -- neatly encapsulates the synthesis of Old and New Testament ideas. For Moses, the central religious duty is to worship one God exclusively. For Jesus, it is to believe in God and his messengers. For Muhammad, these are combined into a duty to believe in one God exclusively -- to deny the existence of any other gods -- and to believe in his messenger.
Of course, modern Christianity attempts, like Islam, to embrace both the Old and the New Testaments, and this has often led to a very Islam-like stance: that one's duty is to believe and obey human religious authorities and deny the existence of all gods but one.
