"If Islam, Buddhism, Zoroastrianism, and all the other world religions are true paths to God, then why did God kill his Son, Jesus, in order to make a way for men to come to him? The very notion is absurd and insulting to God. It
paints a portrait of a God who is just plain cruel. He sent Jesus into the world to live a miserable life of scorn, rejection, poverty, betrayal, humiliation, sorrow, and ultimately, torture and death, in order to create a path whereby men can come to know him. Yet all the while he knew that following the Five Pillars of Islam or the Noble Eight-fold Path could accomplish the same thing. What a waste! Jesus' life—God's plan of salvation— is completely in vain, for the same
result could be achieved by simply adhering to the tenets of any world religion. God is not only cruel but also incompetent for putting into effect the worst salvation plan possible.
But God is not cruel. He is not incompetent. He would not kill his Son needlessly. He would not put into effect a ridiculous or cruel salvation plan for mankind. Hence, religious pluralism cannot be true. This argument does not
show Christianity to be true, but it does show that not all religions can be true, for if they were, then God would not be a God of love."
(Excerpted from http://thegospelcoalition.org/blogs/tgc/2014/03/25/if-all-religions-are-true-then-god-is-cruel/ )