[Petrosius]

If this is the case, then mortal sins cannot cause one to lose their salvation.

You left off: "… even if sin prevents Baptism from bearing the fruits of salvation."

[ealgeone]

Baptism seals the Christian with the indelible spiritual mark (character) of his belonging to Christ. No sin can erase this mark...

If the Roman Catholic is sealed and belongs to Christ and no sin can erase the mark.....how does mortal sin erase the mark?