From one of my favourite books: Mission to Islam and Beyond (pdf file), by Jens Christensen (originally The Practical Approach to Muslims)
29. Muslims (and some Christians) will tell you that as man is limited by the imperfections and evils of sin, a practical shariat like that of Moses or of Muhammed is a necessity. Everybody knows that a state needs laws. What the Muslim and some Christians forget is that the word ‘shariat’ implies a God-given, revealed law for a kingdom of God here on earth. (It makes no difference if that kingdom of God is thought of as identical with the kingdom of Israel or the kingdom of Islam.) That is what our Lord protests against. The kingdom of God is the Kingdom of heaven; it is not of this world, and therefore the subjects of that kingdom must not and cannot blend or confuse its laws with that of any temporal state. Its laws must be purely religious (that is, related directly to God) and unattainable.
‘Why unattainable?’, is the question that both Christians and Muslims ask. The answer is simple. For if sinful man could attain perfection by keeping the law then he is either no longer sinful, or else sin has become a recognised and admitted part of the kingdom of God. The righteousness of the Pharisees was the best, the highest of which the Jews knew, and our Lord said that unless your righteousness exceeds theirs you cannot have a hope of getting into the kingdom of heaven (Matt. 5:20). The unattainable nature of the Christian way of life constantly reminds man of his sinful state and of his need of God’s righteousness.
30. The Jews thought this was a strange, astonishing, new teaching. So it was. The Muslims feel exactly the same way about it. However, until the Jew or the Muslim sees that Christ has unconditionally rejected the idea of a theocratic state as bringing in the kingdom of God, he will not be able to understand our Lord’s attitude towards his shariat.
31. Let me illustrate this very important point in another way. Our Lord said the law and the prophets all hang on these two commandments: ‘Thou shalt love the Lord thy God . . . [and] thy neighbour as thyself’ (Matt.22:37–40). The first of these commandments is taken from Deuteronomy, the second from Leviticus. The second more or less obscure command is found in Leviticus 19:18 and reads like this: ‘Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself’. When the lawyer asked our Lord to define ‘neighbour’, He would, if he had accepted the context in which that commandment is written, have said; ‘The children of your people, whom you contact’. Instead, He made the Jews and (of all people!) the hated Samaritans neighbours. Our Lord took the sense, the idea, in the old commandment and lifted it out of the covenant law which was the state law and applied it universally and personally.
32. When our Lord said, ‘Those of old said such and such, but I say unto you . . .’, He was not just spiritualising the law, as some would have us think. He was actually introducing a new element. He was introducing the consequences of His own preaching when he said: ‘The time is fulfilled, and the kingdom of God is at hand: repent ye and believe the Gospel’ (Mark 1:15). The kingdom of God is the kingdom of heaven.
No theocratic state with its shariat could ever bring it near. Repent, that is, turn your back on that idea, and believe the Gospel, believe that the Messiah, the Son of Man, the suffering and dying servant of Jehovah, has brought the kingdom near, and has introduced God’s righteousness for all men equally, everywhere.
33. The difference between our Lord and the Jews of His time was, concisely, this: The Jews knew that Jehovah had chosen them to be His covenanted people on earth. They therefore thought that they should establish a worldly kingdom of God on earth, probably through the work of the coming King Messiah. Jehovah had given them a shariat together with the Covenant. This they thought was everlasting and was to be applied universally as the law of that universal theocratic state, for by keeping it men became pleasing in the sight of God.
34. Jesus, on the other hand, said that while the purpose of the Covenant with Israel was to establish a special relationship to them, it was not intended to establish a universal theocratic state with a universal law, in which Israel, as a nation, was God’s viceroy on earth. No theocratic state, no shariat, could ever establish righteousness on earth, that is, God’s righteousness. With the rejection of the theocratic state, the law of that state (as the instrument appointed by God whereby men could be wellpleasing in His sight) must also be thrown overboard. Righteousness, God’s righteousness, could only come, as Isaiah said, through the suffering and death of Jehovah’s righteous servant, the Son of Man or, if you like, the Messiah.
35. If you will take this whole idea and apply it to Islam you will find how remarkably applicable it is. Although some of the details will differ (as for example, sabbath-keeping, which is unknown in Islam), yet as such, the picture is clear. Our Lord would be in direct, clear-cut opposition to the Muslims at every step. Nothing they could do would be right, because it is all based on the idea that they belong to the people whose God-given right it is to dominate the world in Allah’s name and thus bring in the ‘kingdom of God’ (although they never use that particular expression) on earth.
36. The Jews thought that they were bringing in the kingdom of God. The Muslims think they are bringing in the kingdom of God—and our Lord says to both: The time is fulfilled. The kingdom of God is at hand; repent and believe the Gospel, which you need as well as every other person on earth.