Won’t a person go to heaven no matter what they believe, just as long as they are sincere?  The Apostle Paul wrote otherwise: “For I testify about them that they have a zeal for God, but not in accordance with knowledge.” (Romans 10:2)  

In this verse Paul admitted that “they” (the Jews of 9:3, “my brethren, my kinsmen according to the flesh”) had a zeal for God: that they were committed, devoted, active, etc. But that zeal was “not in accordance with knowledge” — in other words, it was misguided, and wrong!

There is much talk today about being “a person of faith” — as if the kind of faith the person adheres to doesn’t matter. But the particular of faith one has DOES matter. Faith is only as good as the object in which it is placed! You may have faith that a certain medicine will work — but it must be the correct medicine in order to really help you! Similarly, you may have faith that a certain religion will deliver you — but it must actually be based on the truth to really save.
Thus it is vitally important to consider the words of Jesus, who said: “I am the way, and the truth, and the life; no one comes to the Father but through Me.” (John 14:6)

There is a vital lesson in this passage: just because a person is very sincere and active in their religion does not mean that they are right with God. The Bible makes it very clear that there is only one way to heaven, and that misguided zeal — even a very sincere one — is still sincerely wrong!