The Good Shepherd passage from St. John's Gospel was recently used by my good Protestant (specifically Calvinist) friend to support the once-saved-always-saved doctrine. In particular, John 10:25 ("no one is able to snatch them out of My Father’s hand") was used as a key proof text. A brief response:
The Greek terms for “hearing” (akouōn) and “believing” (pisteuōn) express ongoing actions. True, those in the fold cannot be snatched away, but one who does not continue to hear (one who does not continue to believe) removes himself from the fold. Being assured of my salvation is a pleasant thought, and I totally get why many are drawn to this idea. But it is a false teaching.