Well, it is true in many Catholic circles.
If you are not a part of the Catholic church, one is not a part of the true church.
I have heard this countless times as an Anglican.
Clearly those espousing such a thing are in error.
I've also heard this often from catholics...however the vatican does not agree with them...it states clearly on the vatican website...so Rome does not agree if you don't belong to the catholic church you aren't a Christian...they believe there are Christians in other churches as well.