One can be a “non-practicing” Catholic in a similar manner that it is possible to be a “non-practicing” Jew. A person who is born a Jew but does not practice his faith is still a Jew because he has been born into the Jewish people. A person who is baptized a Catholic (and thus born again) is made a member of Christ’s body, the Church. Baptism imparts a supernatural character upon his soul that can never be lost even if the person apostasizes from the faith and, God forbid, dies unrepentant of mortal sin and goes to hell. Because it is baptism that makes us Christians, because the supernatural character of baptism can never be lost, it is indeed possible but unfortunate that there are people who can be called non-practicing Catholics and non-practicing Christians.
The confusion arises because of a Protestant idea that one is justified by grace through faith alone and that the proof of that is a faithful life. (If a Protestant stops living a faithful life, it is often understood by fellow Protestants to mean that his faith wasn’t real and his justification never “took.” He only thought he was justified, which of course means logically speaking that a Protestant can never be sure of his justification because his only apparent proofs are his thoughts and his current faithful life.)
Our status as Christians is not dependent upon what we do but upon what has been done for us (baptism). We can never change that supernatural mark that makes us Christians, but we can, tragically, refuse to live the Christian life.