Why there are conflicts between religions although they believe in the same Allah?

Hello, my name is Jack and I am trying to get a better understanding of Islam. So I was wondering if you could explain to me why there is even an issue between religions if Islam and lets say Christianity believe and worship the same God and no others?

Dear Brother / Sister,

All of the prophets, from Hazrat Adam to our prophet Hazrat Muhammad, conveyed the true religion to people. The fundamentals of belief, which are the foundations of the religion, always remained the same. However, some decrees about worshipping and worldly affairs, what we call shariah, changed depending on the requirements of the age and needs of people from Hazrat Adam to Hazrat Muhammad. God Almighty sent a different shariah for each community (umma) taking into consideration the lifestyles and interests of the people of each age. The following is stated in verse 48 of Chapter al-Maeda regarding the issue, “To each among you have We prescribed a Law and an Open Way.”

Heavenly religions are in unanimous agreement regarding fundamental beliefs/principles of belief, universal ethical values and the consciousness of worshipping only one god. There are differences in the branches and details of the religion. The differences take place due to the different conditions of the times and places. Therefore, sometimes different sharias came in different ages and sometimes different prophets and different sharias and some new arrangements about details came in the different regions of the same age.

It is known that different religions with different messages and miracles were sent to the communities that had different wrong behavior culturally. The fact that Hazrat Musa (Moses) came to Egypt, which had become a slave of the illusions of the magicians with the same type of miracles the fact that Hazrat Eesa (Jesus) came with miracles regarding medicine in a period when medicine was common and people had unconditional belief in doctors and the fact that Hazrat Muhammad (pbuh) came with the Quran, a unique work of art, and challenged everybody in a period when poets and poems were regarded as literature gods by the Arabs of Jahiliyya are historical and living documents showing the relationship of the cultural and social structure of the environment with religion.
Religions, especially the religion of Islam, draws the lines between halal (licit) and haram (forbidden), lays down conditions to prevent deceiving tricks, unjust profits and cheating regarding the economy; they leave the remaining details to the conditions of the tie and customs of the regions. From this point of view, it can be seen that in the religions, especially in Islam, the last religion, economic conditions like trade, arts and crafts, agriculture were laid down in flexible formulae that can be used in any age.

None of the divine books that were sent before the Quran and whose copies are present today are the original copies of the divine books that Allah sent to His prophets. In the course of time, the original copies were lost and they were rewritten by people. Therefore, some wrong beliefs and superstitions were incorporated into them. For instance, it is a well-known historical truth that the Old Testament (the Torah) could not be preserved by the Jews, who lived as slaves and in exile for a long time after Hazrat Moses and who lost their beliefs for a while and became idolaters; it is also known that the copy of the Old Testament that is present today was known by some clergymen long years after Hazrat Moses and was accepted as a religious book as if it was the original Old Testament. It is obvious that a book that emerged after such a long and complicated period cannot be the same book as the one sent down to Hazrat Moses. Therefore, there are some attributions and slanders that will not behoove to prophets; there are also some decrees that are contrary to the spirit of oneness religion.
The Psalms that were sent to Hazrat David (Dawud) underwent the same process as the Old Testament.
As for the New Testament (the Gospel), Hazrat Jesus did not dictate the revelations that he received. He became a prophet when he was 30 and his duty of prophethood ended when he was 33. He traveled from one village to another, one city to another in order to show people the true way in a very short time, three years. During his last times, he was always traced and followed by the Roman administrators because of the provocation of the Jews. Therefore, he could not find any time or opportunity to dictate the New Testament. As a matter of fact the New Testament books that are present today are called by the names of their writers (compilers) and they look like biography books that include the sermons, preaching and talks of Hazrat Jesus to his apostles.  Besides, the writers are not the first believers, the first apostles of Hazrat Jesus but those who listened to those divine words that came to Hazrat Jesus from them.
There are some differences in the current New Testaments in terms of the narrating style and content. As a matter of fact those books were proved by the decision of a spiritual council that assembled in Nicaea in 325 AD. That council examined hundreds of New Testaments and approved the four New Testaments that are available today that they thought had divine characters and destroyed the others by burning them with the agreement of 318 members.
As it is seen, the principle that Hazrat Jesus is the son of Allah (Allah forbid) was accepted long years after Hazrat Jesus as a result of the decision of a council. Some Christian churches did not accept that decision. Therefore, it is impossible to say that the current 4 Gospels today are the same as the original Gospel that was sent down to Hazrat Jesus.
Since the divine books other than the Quran were falsified how can we believe in them?
We Muslims believe that Hazrat Moses, David and Jesus were sent the divine books called The old Testament (the Torah), Psalms and the New Testament (the Gospel) and that those books did not contain any decrees contrary to the true religion of oneness. However, those books could not be preserved and their originals got lost.
We cannot say that the books that the Jews and Christians have do not contain any revelations that were sent to those prophets. However, it is a reality that wrong beliefs and superstitions were incorporated into them. Therefore, we need to be cautious about them. We accept that the decrees that are in compatible with the Quran are based on revelation. We find it probable that the decrees that are contrary to the Quran were incorporated into those books later. We do not say anything about the decrees that are not in compatible with or not contrary to the Quran. We neither accept them nor reject them. They may just as well be based on revelation or vice versa.
Abu Hurayra (May Allah be pleased with him) said the following regarding the issue: «The People of the Book read the Torah in the Hebrew language and interpreted it in Arabic to Muslims. The Messenger of Allah (pbuh) said to following to his Companions regarding the issue:
"You should neither approve nor contradict the words of the People of the Book. However, say the following: We believe, in Allah and the revelation given to us and to Abraham, Isma`il, Isaac, Jacob and the Tribes and that given to Moses and Jesus and that given to (all) Prophets from their Lord we make no difference between one and another of them and we bow to Allah (in Islam).' (al-Baqara, 136)."

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