A recent Time Magazine Article reports on the Pew Forum on Religion and Public Life which last year surveyed 35,000 Americans, and found that 70% of respondents agreed with the statement "Many religions can lead to eternal life." Even more remarkable was the fact that 57% of Evangelical Christians were willing to accept that theirs might not be the only path to salvation, since most Christians historically have embraced the words of Jesus, in the Gospel of John, that "no one comes to the Father except through me." Even as mainline churches had become more tolerant, the exclusivity of Christianity's path to heaven has long been one of the Evangelicals' fundamental tenets.
This shows how far evangelicalism is from a sound bedrock of confessional truth drawn from Scripture. The Westminster Confession states:
…men, not professing the Christian religion, [cannot] be saved in any other way whatsoever, be they never so diligent to frame their lives according to the light of nature, and the laws of that religion they do profess. And, to assert and maintain that they may, is very pernicious, and to be detested [Westminster Confession of Faith, 10.4]
Robert Reymond writes against religious inclusivism here.
J.G. Vos was a good observer of other religions in "A Christian Introduction to Religions of the World" - he is very perceptive on Islam. He also has a good article on the Good Elements in False Religions. But we must remember that the counterfeit requires to have some resemblance to the truth to be successful. Here is a good article on the exclusivity of Christianity.