Lapsed (fallen-away) Catholic

“Once a Catholic, always a Catholic” is true in the strict sense: Baptism imprints an indelible mark that no lapse can erase, so a person who has not been to Mass in thirty years is still a Catholic who has stopped practising, not an ex-Catholic. Returning is simple in principle — go to Confession, and come back to Mass — and the parable of the prodigal son is the Church's picture of how the return is received. “Fallen-away” is the older phrase; both describe the situation without judging the person, since only God knows the reasons a person drifted.