Why so Many Religions?

“Mankind was one single nation, and Allah sent Messengers with glad tidings and warnings; and with them He sent the Book in truth, to judge between people in matters wherein they differed; but the People of the Book, after the clear Signs came to them, did not differ among themselves, except through selfish contumacy. Allah by His Grace guided the believers to the Truth, concerning that wherein they differed. For Allah guided whom He wills to a path that is straight.” (Quran 2:213)

  So, you see, initially, all mankind was just One Nation, One Religion, and Allah sent messengers to us.

  Some of us chose to follow their messages, and others chose to deny.

  Think of The Ultimate Truth (monotheism) as a giant lake, and each faith is a river which began as pure but was later tainted with untruths.
People began to separate into different groups due to their rejection of truth and denial of clear proofs becoming evident to them from their Lord.

 And the people of the Scripture (Jews and Christians) differed not amongst themselves until after clear evidence came to them.
 And they were not commanded anything more than this; to worship Allah, alone (without making partners with Him in worship) and establish regular devotional prayers and pay the charity due to the poor; and this is the correct Way (way of life and religion). [Noble Quran 98:4-5]

 And be not as those who divided and differed among themselves after the clear proofs had come to them. It is they for whom there is an awful torment.
 On the Day (i.e. the Day of Resurrection) when some faces will become white and some faces will become black; as for those whose faces will become black (to them will be said): "Did you reject Faith after accepting it? Then taste the torment (in Hell) for rejecting Faith." [Noble Quran 3:105-106]

 People lied about the revelations, changed their scriptures and abused and even murdered the prophets whom Allah sent to them.

 And verily, among them is a party who distort the Book with their tongues (as they read), so that you may think it is from the Book, but it is not from the Book, and they say: "This is from Allah, " but it is not from Allah; and they speak a lie against Allah while they know it.
 It is not (possible) for any human being to whom Allah has given the Book and Al-Hukma (the knowledge and understanding of the laws of religion, etc.) and Prophethood to say to the people: "Be my worshippers rather than Allah's ." On the contrary (he would say): "Be you Rabbaniyun (learned men of religion who practice what they know and also preach others), because you are teaching the Book, and you are studying it." [Noble Quran 3:78-79]

 Prophets of Allah only ask people to worship Allah, as One God without partners. They would never ask people to worship them or anyone or anything else. Allah tells us in the Quran:

 Nor would he (a messenger of Allah) order you to take angels and Prophets for lords (gods). Would he order you to disbelieve after you have submitted to Allah's Will?
 [Noble Quran 3:80]

 Man made religions
 an abomination before the Lord and will never be accepted.

 Do they seek other than the religious way of life of Allah (the true Islamic Monotheism worshipping none but Allah Alone), while to Him submitted all creatures in the heavens and the earth, willingly or unwillingly. And to Him shall they all be returned. [Noble Quran 3:83]

 Allah will only accept true submission, obedience and in purity and peace to His commandments.

 And whoever seeks a Way of life other than submission and surrender to Allah's Will (Islam), it will never be accepted of him, and in the Hereafter he will be one of the losers.
 [Noble Quran 3:85]

 To believe in Allah and follow His Commandments has been the message of all prophets of monotheism.

 http://legacy.quran.com/3/80-85