Not every religion can be true. Most are mutually contradictory. Either one is true and the others are false, or they are all false. Either Christ is who he said He is or He is not. If He is not, then He was lying, He was sincerely deluded, or the stories were all made up about Him.
If Jesus is who He said He is, then Christianity is true and He is the only way to God.... so if you believe in Christ, then you also believe that only Christians can go to heaven. Which is a bit hard on all those people born before Christ who were promised all kinds of stuff by God. Was God lying? Perhaps only people born after Christ who are christians can go to heaven and only Jews before Christ? Schaefer addresses the question of those who have not heard about Christ and so have had no opportunity to become Christian, later.
Note that not all Christians take the line that only Christians go to Heaven, they interpret John 14:6 as referring to generally living a good life as described by Jesus rather than a literal "you must be a Christian". They'd point out that Christianity did not exist at the time - Jesus was Jewish.
We are saved all this trouble if we just accept that Jesus was deluded (or mis-quoted, or fictionalised to some extent.) Certainly the Bible has Jesus saying a lot of vague mystical metaphorical things which can be interpreted to mean that he says he is God. But he also says "I am a vine" (John 15) "I am a door" (John 10)... was he either of those things? Clearly believers will pick and choose their interpretations to fit their beliefs. Perhaps it is the modern followers of Jesus that are deluded?