How can a christian be a catholic?

Answer 1

 The bible states that if anyone preach any other gospel than the one I preached let him be cursed..galatians, and also in IIjohn, if anyone bids godspeed to any one who brings a different message he shall be a partaker of his evil deeds.

Although it well could be rare, there may well be Catholics who indeed are Christian who have not bought completely into the teachings of the Catholic church. We can't work from the assumption all Catholics think and believe the same things.  Salvation is to those who believe the gospel, and it is possible there are Catholics who do and do not believe the false claims of salvation extant within the organization.