Some how I missed posting on this subject so I think I'll give a long explanation on what I believe on this subject. To get this out of the way, the question "are other religions correct?" usually turns into "can other religions get to heaven?" so that is what I am going to post on.
First we must define a few terms:
Sola Fide- Sola Fide literally means "faith alone." This doctrine is held by almost all Protestant denominations and to a certain extant the Catholic Church. This doctrine DOES NOT mean that one has to confess the Apostles Creed and go to church every Sunday to be saved. Faith means a relationship with God, not acknowledgment that God exists (that is belief). Thus when one says people are saved by "faith alone" that means that it is only by right relationship with God that one receives God's saving Grace as opposed to being by ones good works
Sola Gratia- Literally means Grace Alone. Grace alone means that only God's grace gets us into heaven. This is more of a protestant just to warn everyone. God does not care about our works. There is no way to earn heaven because we cannot obtain perfection. "For all fall short of the glory of God" as Paul says in Romans. To earn heaven one would have to obtain the perfection of God and nothing less. They would have to love perfectly, give perfectly, and serve perfectly. Since all these things are impossible one one thing remains, God's grace. God wants for us to be in right relationship with him so he extends his grace.
Sensus Divinitatis- this phrase means "sense of the Divine." Described by John Calvin, it means that all people have a sense of who God is because he is their creator.
Ok since that is one of the way we can get moving. Several early Church Fathers have indeed left the possibility for other faiths to get to heaven. Clement of Alexandria and Origen both briefly discuss this issue.
Third John 10:16 says: "
I have other sheep, which are not of this fold; I must bring them also, and they will hear My voice; and they will become one flock with one shepherd." Other passages can by used such as the story of Melkizedek in Genesis 14 or Jesus' parable about the sheep and goats in Matthew 25 but I think this one is the best. Christians usually hold that Christ's death not only allows the correction believers relationship with God, but all Creation's relationship with him. Thus to answer the question, if we hold that all people have a "sense of the Divine" and that God is "abounding in steadfast love" as the Psalms say then I think other religions have elements that are correct. All people have a "sense of the Divine" which means that all people in some way are seeking some kind of relationship with him (either good or bad). If a person of another religion has a right relationship with God and if we hold that it is by God's "grace alone" that we are saved than see no reason why other religions cannot have elements of truth in them. In answer to this question the best answer I usually have is Christianity is the best way and surest way, but not the only way.
I should probably note that on this issue (and many others) I am in no way considered a conservative Christian. This is my theology on this issue and many would disagree, but I think it could hold up enough to some Biblical investigation. Being a fairly liberal (religiously that is) Protestant there would be vast portions of Christianity that would disagree. Actually the best and truest answer is "God knows." God is the judge of such things and only he knows all these things. But if one really wants a answer that says yes or no on the subject then the above is mine.