John 14:6 Jesus answered, "I am the way and the truth and the life. No one comes to the Father except through me." This is the reference you get your argument from. This same Scripture sounds to me like the way of Jesus is the way to the Father and heaven. Do you see the difference? Jesus's way was to accept people and forgive them and to pray for enemies and to follow the laws of God and his ways which we know of as the commandments of Jesus of which there are about 50. The difference is that some people think praying the sinners prayer in a moment of passion is enough to enter heaven, and I think that people who actually follow the ways of God whether it be in Christianity or not, are the one's who are justified by Jesus. Because according to this Scripture, Jesus is the way. So to come to the Father one must follow the way of Jesus.
Islam for example does not recognize Jesus as the son of God. They say he was nothing but a prophet. They are denying everything he is about.
Furthermore, there is the unforgiveable sin. blasphemy of the Holy Spirit. To deny JEsus is to deny the Holy Spirt.
but he who blasphemes against the Holy Spirit never has forgiveness, but is subject to eternal condemnation