A reader wants to know what makes something Catholic and not Christian? Are these different things, or the same thing?
The answer is simple.
The Catholic Church is the Church Jesus Christ established, and full communion with it is full communion with Christ. Only the Catholic Church is fully Christian; other churches are partially Christian and are becoming either more Catholic or less Catholic as the years go by. Only the Catholic Church teaches the hard truths that many Protestants ignore - the sanctity of life, the evil of contraception, the indissolubility of true marriage, the sin of sexual activity outside of marriage - perverse or otherwise, the Sacramental nature of the Incarnation, salvation by grace through faith made manifest by works, and the awesome and confounding love of God even for sinners. Everything else that's Christian but not fully Catholic falls shy of this.
This does not mean that your typical Catholic is any more holy than many Protestants, Eastern Orthodox or even an odd atheist here or there. It does mean that only in the Catholic Church does the fullness of the Body of Christ subsist, and to reject even part of its teaching is to leave yourself cut off from full communion with that Body.
All baptized believers are baptized into the One True Church. And only by nourishing herself with the Living Waters that flow from her can we hope for eternal life. This does not mean the "unchurched" are damned, but it does mean that only from Christ does salvation flow, and that if the unchurched or non-Catholics are saved, it is by His grace and their participation in it that He saves them, and that they are judged according to their proper lights.