"Are Christians the only ones who can access Heaven?"
I do not know.But contemplate verses below.
Colossians 2:8
See to it that no one takes you captive by philosophy and empty deceit, according to human tradition, according to the elemental spirits of the world, and not according to Christ.
I have prepared for you verses regarding salvation.
Look there:http://www.openbible.info/topics/salvation
Good luck
