I heard that the bible and tora used to be faith-books of Islam. The dilemma was that the human changed those books after their needs. After a time Allah wanted to send down the unchangeable and final book: the Qur'an. If all this right what I wrote, I really don't understand why Allah did not send the Qur'an at first and made the first faith-book unchangeable.
بسم الله الرحمن الرحیم
I think you'd better say: " Why did not Allah send his last prophet, Mohammad (S), at first? then I will tell you that, as you know, Allah has gradually developed Islam since the advent of Adam (s). The more the people were growing and the more aspects of their lives needed to be explained and reformed, the Almighty Allah sent his prophets and divine books with them. To prove my claim I would like to refer to this verse from the noble Qur'an as such (According to this verse, the only existing religion is submission to Allah):
إِنَّ الدِّينَ عِنْدَ اللٌّهِ الإِسْلاَمُ وَمَا اخْتَلَفَ الَّذِينَ أُوتُوا الْكِتَابَ إِلاَّ مِنْ بَعْدِ مَا جَاءَهُمْ الْعِلْمُ بَغْياً بَيْنَهُمْ وَمَنْ يَكْفُرْ بِآيَاتِ اللٌّهِ فَإِنَّ اللٌّهَ سَرِيعُ الْحِسَابِ
“Surely the religion with Allāh is al-Islām. And those who have been given the Book [i.e., the Christians and the Jews] did not show opposition but after knowledge had come to them, out of envy among themselves. And whoever disbelieves in the verses of Allāh, then surely Allāh is quick in reckoning.”
So, it seems logical to have different various religions and divine books, Although one might ask: "why did not Allah created the grown human at first?" which can't be answered here thoroughly. but God's will was to educate the human being step by step, in fact he sent the final prophet along with his last divine book when the humanity was ready for it.
Assalaamualaikum brother. As you know Allah sent messengers over time and sent down books on some of them, four of which we know are The Psalms (Zuboor), The Torat(Taurah) , The Gospel(Injeel) and The Quran.
If you see, the books that were sent before had a messenger that would be coming in the future, and that period of time wasn't the end of revelations from Allah. The Messengers used to confirm what came before them, used to explain some new laws of Allah and give tidings of the next messenger.
And [mention] when Jesus, the son of Mary, said, "O children of Israel, indeed I am the messenger of Allah to you confirming what came before me of the Torah and bringing good tidings of a messenger to come after me, whose name is Ahmad." But when he came to them with clear evidences, they said, "This is obvious magic." Al Quran [61:6]
It is He Who has sent down the Book (the Qur'an) to you (Muhammad) with truth, confirming what came before it. And he sent down the Taurat (Torah) and the Injeel (Gospel). AL Quran [3:3]
As the laws at different times were different, for example AL-ANFAL(the spoils of War) were Haraam for all the previous Ummah except for The Ummah of Muhammad (sallallaho alayhi wassallam) and similarly Lailatul-Qadr is the power night exclusively for our Ummat.
As for your question, Allah is capable of everything. He could have made the Psalms, Taurah or Injeel unchangeable but Allah did not guarantee that (out of his infinite knowledge) (the reasons could be to make the hypocrisy of the people who changed verses apparent to others, Allah knows best). But for the Qur’an, it is the last revelation till the Day of Judgement and there will be no messengers to come and bring the truth, so Allah guards this book Himself so that we have a pure form of guidance still available. That’s the mercy of Allah (SWT) .
The laws were different during other periods and hence there were different books with some different laws. Baarakallahu Feekum.
Assalam Alaykum. Brother, Allah sent humans to Earth so He may test them. Quran, 72:16-17 http://quran.com/72/16-17
And [ Allah revealed] that if they had remained straight on the way, We would have given them abundant provision
So We might test them therein. And whoever turns away from the remembrance of his Lord He will put into arduous punishment.
So this is the reason for the first books being, 'changeable'. However, without a clear book, which cannot be changed, humans would be lost forever. (By continuously altering the books to suit their needs)