Why don't they use the same Method?

"Those who follow the apostle, the unlettered Prophet, whom they find mentioned in their own (scriptures),- in the law and the Gospel;- for he commands them what is just and forbids them what is evil; he allows them as lawful what is good (and pure) and prohibits them from what is bad (and impure); He releases them from their heavy burdens and from the yokes that are upon them. So it is those who believe in him, honour him, help him, and follow the light which is sent down with him,- it is they who will prosper." (Surah 7:157)

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Notice the verse is referring to the Jews and Christians (whom they find mentioned in their own (scriptures),- in the law and the Gospel) WHOM THEY WILL FIND. now alot of people have fallen into confusion within their own understanding who think that Prophet Muhammed Pbuh should be mentioned by Name or there should be some very clear and indisputable informations about him in the previous scriptures

According to the understanding of Dr Shabir Ally The Quran is calling Jews and Christians to use interpretive methods with which they are already familiar with. To see if those interpretive methods points towards Prophet Muhammed Pbuh as these interpretive methods pointed the Christians to Jesus Pbuh.

What methods did the Christians use? They use typology they will say  Jesus is not  mentioned in the Old Testament by name but he fulfilled some types of things which were said in the Old Testament like there were sacrifices and now Jesus has become the ultimate sacrifice.. 

They use the idea of the fuller sense of scripture, they would say something was mentioned there but it would not have been understood at that time. It when Jesus came it became very clear, he's bringing out now the fuller sense of scripture 

They use also Midrash Christians may not think of themselves doing this today but the New Testament writer like Matthew and other have done this, what they have done is they have looked at an Old Testament passage and they have said in advance of Jesus arriving on scene that there was going to be a Messiah coming. No particular s rapture says that here and now a Messiah will come and his name will be Jesus but they are looking for some clues in the Old Testament and they are seeing that is fulfilled in Jesus 

Moreover they use an interpretive  method known as  pesher what they take Jesus as he is and then go back to the Old Testament to see if perhaps there is some sort of indication no matter how vague about this man that we know to be Jesus.

So this could mean the Quran is really asking Jews and Christians use these methods with which you are already familiar and with which you are already for example identified Jesus as a man of God, Prophet and Messiah of God.. 

If you have used those methods to identity Jesus what prevents you then from using the same methods and recognising Muhammed Pbuh is A Prophet and Messenger from God, and there are indication of those in the Old and New Testament As many of use are familiar with.