Why Islam was Given to the Mankind
There is causal relation in anything, including the establishment of Judaism, Christianity, and Islam.
Before Muhammad was given words of God, he encountered Judaists and Christians in his home town and in his travelling for trade as far as Syria. Even the words God gave to Muhammad included many references to Judaism and Christianity. So, just to make Muhammad more pious and faithful to God, God could have told Muhammad to follow Judaism or Christianity with more enthusiasm. But, God gave Muhammad a new religion Islam.
It suggests that God was not satisfied with Judaism and Christianity at least around 600. It can be also thought that God had not been satisfied with Judaism around AD 1, so that God gave His Son to the Judaist community or to the world, allowing for establishment of Christianity. But what were main factors of God's dissatisfaction with Christianity around 600?
Till about 600 when Muhammad started his mission, Christianity had prevailed in the Roman Empire to become the state religion of the Empire and it had survived the era of turmoil after the fall of the Empire and the Great Barbarian Invasion. Rather, the Vatican had further consolidated its organizational system and fixed theology though these centuries.
In this process of development of the Vatican, it expelled some schools of Christianity as heresy while acquiring its own territory and strengthening a tie with the Merovingian dynasty that established its power base in territories covering today's France and Germany. There must be something that made God get dissatisfied in these movements of the Vatican.
The Vatican did not allow existence of various schools of the Christian theology, while adhering to the theory that Christ Jesus was God. The Vatican showed desire to govern its own territory and strengthen its political tie with worldly kingdoms. These movements were against the original teaching of Christ Jesus. And, even for this reason, it is thought that the Roman Empire collapsed while it adopted Christianity as its state religion.
Put simply, it became clear that Christianity failed around 600, so that God gave words to Muhammad who then established a new religion Islam.
However, although 1400 years have passed since the start of Islam, God has not given another religion to the mankind. It might mean that God is satisfied with Islam, or at least satisfied with Muslims more than with Judaists and Christians.
Anyway, there must be reasons that God gave Judaism, Christianity, and Islam to the mankind and will not give another religion any more. This is the most significant issue for the mankind in these 4000 years since God chose Abraham.
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Mat 8:20 And Jesus saith unto him, The foxes have holes, and the birds of the air have nests; but the Son of man hath not where to lay his head.