What should I say to my non-denominational friend

Hello guys, tomorrow me and my non-denominational friend have arranged to talk about Catholicism. He keeps saying why can’t we all just “unite” as Christians, something you hear from a lot of non-denominationals. Although I do have a few points I’ll bring up, I wanted to ask what’s the best thing to say to counter this way of thinking, some passages form Sacred Scripture would also be very helpful, thanks.