(Read the series so far.)
As far as I can tell the Roman Catholic Church now teaches that the most likely way to go to hell is to believe Roman Catholic doctrine. Allow me to support this from a series of quotes from the Catechism of the Catholic Church (CCC).
The CCC teaches that Protestants who disbelieve the doctrine of the Roman Catholic Church are likely saved:
CCC-818: However, one cannot charge with the sin of the separation those who at present are born into these communities [that resulted from such separation] and in them are brought up in the faith of Christ … . All who have been justified by faith in Baptism are incorporated into Christ. (Brackets in original.)
CCC-819: Christ’s Spirit uses these Churches and ecclesial communities as means of salvation, whose power derives from the fullness of grace and truth that Christ has entrusted to the Catholic Church.