People "go to hell" because that is the punishment for sin.
I put "go to hell" in quotes because we could have an entire thread discussing what that actually means... let's not do so in this thread.
It is only by God's grace and mercy that we aren't all headed there. Jesus, God incarnate, is the only One Who could make an offering which would satisfy the justice of God and make our redemption possible.
Certainly there are believers who died without understanding the fullness of Who Jesus is and why His sacrifice was sufficient; we don't become Christians and immediately receive a download of understanding. We do, however, receive the Holy Spirit, Who brings us into fullness of understanding over time.
Many of us have incomplete and even incorrect ideas about Jesus; that doesn't necessarily make us un-Christian. However, actively rejecting key truths about Him puts us in jeopardy. We do well to continually seek to learn, understand, and live out our lives as an expression/reflection of Him.