"Did you think that We had created you in play (without any purpose), and that you would not be brought back to Us?" (23:115)
Belief in the existence of an All-Knowing and All-Wise Creator necessitates that He created us for a very important reason:
"And I (Allaah) created not the jinns and humans except they should worship Me (Alone). " (51:56)
If mankind was created to worship and serve the Creator, certainly He would have explained to His creation how this should be done. However, many people today believe that Allaah neglected sending down one perfect religion and way of life that is pleasing to Him. They claim that the ways to God are many, and that it is up to the individual to make up his or her own "personal path to God." Since they do not beleive that their Creator ordained one set path for them to follow, they also conclude that mankind is not answerable to Him regarding whether or not they followed this one way.
Interestingly enough, if one of these people were to buy something mundane like a piece of unassembled furniture and found that there was no assembly guide in the box, he or she would almost certainly return to the store it was bought from and demand one. Amazingly, people expect to receive a reference guide for their material goods, but they do not expect any guideline to be send down from their Creator in regards to their worship, lives and ultimate destiny.
Amazingly, many people believe that God knows everything, yet still proceed to put together a philosophy, religion, or way of life for themselves. Certainly, this belief necessitates that this All-Knowing God has created us without ordaining a way of life for us, or that He has, but that mankind can put together their own way of life that is superior to His.
"They follow nothing but conjecture and what their own souls desire, even though there has already come to them Guidance from their Lord." (52:23)
Contrary to the principles of Pluralism, the Creator سبحانه و تعالى has clarified His will that mankind adhere to to His one, chosen way:
"And, [moreover], this is My Way, which is straight, so follow it; and do not follow [other] ways, for you will be separated from His way. This has He instructed you that you may become Muttaqeen ." (6:153)
Allaah سبحانه و تعالى said that this is "My Straight Way," and that mankind should "follow it." He did not say, "These are My Multiple Ways."
No matter how attached people might be to the religions and ways they have inherited, it would not make sense to believe that the Creator recognises any religion except the one He has revealed:
"And whoever seeks a religion other than Islam, it will never be accepted of him, and in the Hereafter he will be one of the losers." (3:85)
It would not be befitting for the Creator, Who is One, to legislate many contradictory truths, religions and ways, and be an author of confusion. Consequently, Allaah سبحانه و تعالى forbade mankind from following anything other than the way that He has ordained for them, saying: "and do not follow [other] ways." The wisdom behind staying away from other paths is clear, as the remaining part of the verse clarifies: "for you will be separated from His way."
Believing that the Creator ordained many contradictory ways to worship and obey Him does not make logical sense. This belief entails that the Creator created and sustains everything with absolute wisdom, yet failed to outline a complete path which He wills His creation to follow. It would go against the wisdom of the Creator to ordain many ways, because pluralism causes people's concepts of good and bad to become unclear. Pluralism also causes people's understanding of who the Creator is and what He wants from His creation to be obscured.
Likewise, under these pluralistic circumstances, people's rights become squandered, as no set system of values exists which would be able to guarantee people certain basic, irrevocable rights. As a result of this approach to life, all beliefs and values become subject to change at any time.
In short, pluralism breeds confusion and contradiction in the most basic matters of life, and causes mankind to be divided. Division of thought and ideology causes splitting and disputing even if it might appear that these people are united and tolerating each other's different ways.
For this reason, the Creator سبحانه و تعالى described the condition of those who would differ in fundamental matters of life after having clarified His guidance in these regards as having failed to use their God-Given sense of reasoning correctly:
"You would think they were united, but their hearts are divided, that is because they are a people who understand not. " (59:14) 
 See: Fear Allaah Wherever You Are (Taqwa) - http://www.muftisays.com/forums/12-virtues/7703-fear-allah-wherever-you-are-taqwa.html
 Haneef Oliver, Sacred Freedom: Western Liberalist Ideologies in the Light of Islam