A Christian is someone who has repented of their sins and received the Lord in their heart by faith, recognizing there is nothing they can do to earn or work for their own salvation, that Jesus paid the entire penalty on the cross.
The RCC doctrine is against all that. RCC Doctrine teaches that a person must work to earn Salvation, that there is some sins that the penalty for them has to be worked off in purgatory, etc.
An individual RCC person might believe in the Biblical plan of salvation, but they would not be happy in the local RCC Church nor would they like what they keep hearing from the priests or laity for it would clash with what they read in the Bible more and more
Yes, an individual who attends a RCC Church can be a genuine Believer, but there is no way someone who holds to RCC Doctrine is, because RCC doctrine is against what the Bible teaches on almost all of the doctrines that have to do with Salvation, starting with "faith plus works", whether Jesus death on the Cross paid the entire penalty for sins, is Christianity the ONLY saving faith. (I was shocked to read in the Chatechism that Islam can save you if you are sincere! I am away for work or I would dig that one up, I was stunned.)