If it is in John, why isn't in in Mark? Don't you find that strange?
If you were writing about a man who claimed that he was the only way to salvation, would that not be something you would mention? If he said that, it would certainly be the most important thing to report if you were going to tell someone about him. But the Gospel of Mark never says that. If he said that it is just absolutely bizarre that Mark never tells us. But multiply that strangeness. Along comes the Gospel of Matthew, where someone decided to write another Gospel specifically to include things that the Gospel of Mark missed. And does the Gospel of Matthew tell us that Jesus said this? No. And the Gospel of Luke around the same time, a Gospel that claims to have done a lot of research to give us a more complete story of this person. Does the Gospel of Luke tell us that Jesus said this. No.
If Jesus said this can you believe that three Gospels just neglected to mention it?
Very very strange.
Unless of course Jesus didn't actually say that.