If God only needed one messenger, then God would have only sent one messenger. Otherwise it is like God made a mistake with all the other messengers, and that doesn't make sense.
ok, I can answer this in sha Allah.
Why does God need multiple messengers? Because different kinds of people respond to different kinds of messages. Some people relate very well to the Quran, but I relate more to the Old Testament. The short version of each religion/message is that Islam is about submission to God, Christianity is about faith in a particular interpretation of God, and the Old Testament is about wrestling with God which means questioning everything until one finds one's way to God. (Judaism is none of these, it is submission to the rabbis.)
Your premise is this:
God sent messengers to many communities and thus, He is faulty - and if God was perfect then He would in His infinite wisdom, send only one messenger. For all people's.
My answer:
God did indeed send messengers to many communities, a messenger from their own people... let's take the example of the Old Testament Prophets and Messenger's shall we?
How many Prophets and Messengers did the Children of Israel kill, exile or ridicule in the worst ways? Quite a few right? Was it the Messengers who were at fault? or the people they were sent to for disobeying God's messengers, and worse - killing them, exiling them, etc?
The Children of Israel were at fault.
How about Jesus pbuh? Was he not only sent to the lost sheep of Israel? And how did they treat him? They tried to crucify him, but God raised him to HIMSELF this is when the covenant was broken... the final straw... God takes away the land of the Israelites who were descended from Abraham pbuh and gave the covenant to their brethren, from the line of the first born of Abraham, Ishmael pbuh - through his descendant, Muhammad pbuh.
Up to this point, where Jesus was sent - God had promised the Israelites that HE will continue to bless them and give them prophets and messengers to keep them guided but with the children of Israel wanting to kill the Messiah they were awaiting, Jesus pbuh, they'd totally rejected God's promise and covenant and this was the final straw that broke the camels back, so to speak. Hence, it was time to make HIS word heard all over the world.
Enter the descendant of Ishmael, pbuh, - Muhammad pbuh - who was sent as a mercy to mankind, and not just a people - tasked with ensuring the measures were put in place to get the true creed of the Abrahamic tradition to the four cardinal corners of the planet - and this is exactly what happened.
So when you claim God made a mistake - I would have you understand that God makes no mistakes but repeatedly gave the children of Israel chances over and over again to keep the covenant intact - which they failed in doing. This covenant had to pass - onto the descendant from Abrahams lineage through Ishmael his first born son - and thus we see the promise fruit all over the world today.
Christianity of today is a Pauline invention as the disciples of Jesus pbuh were hunted down by the very same Paul who was a mercenary hired to hunt down those early companions of Jesus pbuh until they were either exiled from the holy land by themselves for their own safety or killed in the process by the long arm of the Judeo-Roman establishment of the period.
Christianity thus, never saw a gospel of Jesus in any written form, but was an oral tradition which became extinct and thus the four authors of the New Testament - Matthew, Mark Luke, John are marked with an X after their name as no one knows who these men were in history... I ask you, would you believe in a text book authored by someone who didn't leave you his last name?
As for the Torah, it was once a complete work, but the Torah disappeared and was no longer a circulated holy work when the children of Israel were in bondage in babylon... it wasn't until one Prophet had to re-form it from memory without no one to check his accuracy of statements made within - came to pass and Jesus pbuh came to recorrect the corruptions the children of Israel had entered into it or the laws they removed from it - that Torah is still very much in use today as corrupt as it is... it still allows the children of Israel to commit usury on other peoples and even their own - a grave sin.
Why all these corruptions, you ask? well did God ever say He will preserve their holy books? NO. He didn't say so in the OT, nor did he mention it in the NT...
...Yet, the Qur'an which is a guidance for all of mankind contains this verse:
We have, without doubt, sent down the Message; and We will assuredly guard it (from corruption). - 15:9, Qur'an.
And more than 14 centuries have passed and not a single letter has been added, removed or changed in it. Islam has spread to all nations and the theology sound, One God - worship HIM alone - ascribe no partners to HIM...
Whereas with Christianity, varying theologies by which any Christian can, if they choose to, interpret it as they wish... and with NT's with books added, removed and a lot more besides, as well the fact that the four attributed authors of it, are mysteries in history...
...The Torah, corrupted due to the greed of the rabbi's.
But only the Qur'an remains intact, audibly recited to heart by millions of Muslims around the globe.
The Prophet pbuh last sermon specifically addresses the issues of racism and makes this a thing of the past as no nation, no race no creed is to be left out of the fold of Islam should they choose to embrace it - this is for all of humanity, and as such we find it is the fastest growing faith in the world - as God intended.
Soon, very soon the numbers of Muslims will swell to beyond what any faith has ever seen before in the history of mankind and that is the promise of the revelation. Despite the Wests attack on the middle east where millions of Muslims have been murdered in cold blood.
I have just demonstrated to you how your ideas about religion were half correct and then gave you the contextual dissemination of historical and religious nuances which led us to this point in order to prove to you that your ideas were only half wrong because they were OOT.
Scimi