Protestant
Once saved, always saved — is that biblical?
Scripture presents salvation as an ongoing relationship of grace that can be forfeited through unrepented mortal sin, since Paul himself warns believers to work out their salvation 'with fear and trembling' and describes the possibility of a shipwrecked faith.
Catholic teaching
The Church teaches that justification is a real, transforming gift received in faith and baptism, but that the justified can, through grave sin freely chosen, lose the grace of God; hence the call to persevere and the provision of the sacrament of confession for those who fall.
Biblical foundation
Paul tells the Philippians to 'work out your own salvation with fear and trembling' (Phil 2:12), warns the Corinthians that he disciplines his body 'lest... I myself should be a castaway' (1 Cor 9:27), and Hebrews warns of the terrible possibility of falling away after being enlightened (Heb 6:4-6).
Scripture references
Historical evidence
The early Church's practice of penance for grave post-baptismal sin, attested already in the Shepherd of Hermas in the second century, presupposes that believers could indeed lose the grace of salvation and need restoration, contrary to a later doctrine of guaranteed perseverance.
The Church Fathers
“I have heard, sir, from some teachers, that there is no other repentance except that which took place when we went down into the water... But he said to me... there is a second repentance given.”