I never quite understood it myself but I think it is based on two related matters, the second of which, as it were, explains or expands on the first:
1. Jesus speaks of behaviour which puts the believer so beyond the pale that s/he he is capable of losing his/her salvation, eg: blasphemy against the Holy Spirit (Matt 12: 31-32; Mark 3:28-29; Luke 12:10); failing to forgive others (Matt 6:14; 18:23-35; Mark 11:26); and not dwelling in Him (Jn 15:6); similarly John writes of a 'sin which leads to death' (I Jn 5:16).
2. It is pretty logical (to me at least) that one can choose voluntarily and knowingly to apostasize, to commit a repudiatory breach of one's relationship with God, that goes far far beyond 'normal' sinning and amounts to an utter rejection of salvation. I can't understand why anyone would want to do that, but it is nevertheless possible.
[cp with just about everyone else whilst I was looking up the references!]
1. Jesus speaks of behaviour which puts the believer so beyond the pale that s/he he is capable of losing his/her salvation, eg: blasphemy against the Holy Spirit (Matt 12: 31-32; Mark 3:28-29; Luke 12:10); failing to forgive others (Matt 6:14; 18:23-35; Mark 11:26); and not dwelling in Him (Jn 15:6); similarly John writes of a 'sin which leads to death' (I Jn 5:16).
2. It is pretty logical (to me at least) that one can choose voluntarily and knowingly to apostasize, to commit a repudiatory breach of one's relationship with God, that goes far far beyond 'normal' sinning and amounts to an utter rejection of salvation. I can't understand why anyone would want to do that, but it is nevertheless possible.
[cp with just about everyone else whilst I was looking up the references!]