The scriptures make it clear that its possible to be zealous for God but lost nonetheless.
Paul wrote of his countrymen:
"My heart's desire and my prayer to God for them is for their salvation, for I testify about them that they have a zeal for God but not in accordance with knowledge" (Romans 10:1,2).
Did you notice that? Paul's desire for them was that they be saved. Obviously they weren't - even though their zeal for God was unquestioned.
Consider what Jesus said in reference to the judgment day:
"Many will say to me on that day, 'Lord, Lord, did we not prophesy in Your name, and in your name cast out demons, and in Your name perform many miracles?' And then I will declare to them 'I never knew you, depart from Me you who practice lawlessness'" (Matthew 7:22,23)
Again, plenty of zeal but lost.
What Jesus said earlier in Matthew 7:
"Not everyone who says to Me, 'Lord, Lord' will enter the kingdom of heaven but he who does the will of My Father who is in heaven" (Matthew 7:21)
So its actually doing the will of God that gets you into heaven according to Jesus, not just believing you are. But to do the will of the Father you have to know the will of the Father. Study the New Testament yourself - carefully. Don't just take men's word for what it says (including mine!). There's eternal life at stake.