"Enter through the narrow gate; for the gate is wide and the way is broad that leads to destruction, and there are many who enter through it. For the gate is small and the way is narrow that leads to life, and there are few who find it." Matthew 7:13-14
Why is Christianity so narrow? Because truth, by very definition, is narrow. The equation 2 + 2 = 4 is narrow. The answer cannot be anything other than 4. There is no such thing as "your truth" and "my truth"; there is only the truth. Christianity is narrow because Jesus, the very Person many of you self-professing so-called "Christians" claim to love and follow, said that the way to God is narrow. "I am the way, and the truth, and the life; no one comes to the Father but through Me!"
Jesus declared that He was the only way to salvation, to right standing with God. You self-professing so-called "Christians" who question this, who deny this, who say there are many ways to God that even a devout Muslim, Hindu, Mormon, Jehovah's Witness, etc., may get to Heaven; you call the very Person you claim to follow and obey a liar! Jesus said "the way is narrow," and you say that it is not. Jesus said "I am the way" and "no one comes to the Father but through Me," and you say that He is not, and that people can come by any way. You make Jesus to be a liar! Why? Because you are not a Christian! You do not belong to Him! You are not the Father's child!
You claim Jesus for your own, yet reject everything He said and stood for. You claim He was wrong about creation. You claim He was wrong about Adam and Eve being the first humans. You claim He was wrong about the flood. You claim He was wrong about upholding heterosexual relationships and condemning everything that did not conform to this. You claim Jesus for your Saviour and God, yet spew the denials of your true father, the Devil. Unless you repent and start believing what the Saviour said, when you die you will spend an eternity in Hell. You profess to be a Christian, but you are anything but. A Christian obeys his/her Master. A Christian does not deny what his/her Master has said and make Him out to be a liar. You call yourself a Christian, but you were sold on a false Gospel and you have never been regenerated, born again, and saved.
If you believe that any religious person can get to Heaven by observance of their religion, then you are not a Christian. If you believe the way to salvation is not narrow, then you are not a Christian. If you believe that Jesus is not the only way to the Father, then you are most definitely not a Christian.
You engage in the Devil's most successful motto: "If you can't beat 'em, join 'em." The Devil's greatest victory is in having false converts and non-Christians claim to be and pretend to be Christians so that it gives the rest of the world a bad taste of what it means to be Christian. Many cults and sects claim to be Christian, yet deny the very words of Scripture and teach doctrines of men and doctrines of demons. Oh, the condemnation that rests upon your head for the day of judgment!
Christianity is narrow because truth is narrow. Truth cannot be untruth. Truth is narrow because God is narrow. God is absolutely holy and cannot be anything but. If God provides a way, then that is the only way and everyone who comes to Him must come the same way: by faith in Christ Jesus as Messiah the Saviour.