I recently started to someone that there is only one way to heaven and that is in Christ. It got some shocks, but, I was simply stating what The Scriptures say. You are either not part of God's work of substitution for Sins and destined for destruction/punishment. Or you are in Christ and partaking in his atonement for your sins being heaven bound.
Any Orthodox and solidly biblical Christian would say the same thing. If you do not believe in what is in The Bible about how you are saved you are not a Christian. The bible clearly states the exclusivity of the way to heaven. In fact, Christianity started off being called simply, The Way.
I recommend clicking on the how to be saved poster on the right hand sidebar for more information on this very topic.
