Last night I was listening to a debate between a Catholic apologist and a Baptist apologist. The Baptist kept using Jn 6:39: "and this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up at the last day" as a proof text for the Protestant belief of Once Saved, Always Saved.
Yet had the Baptist read all of John's Gospel, instead of taking random verses out of context to support his preconceived notions, he would have read Jn 18:7-9 where we see that it doesn't mean that at all! The context is the Pharisees and men coming to arrest Jesus:
"Again he asked them, 'Whom do you seek?' And they said, 'Jesus of Nazareth.' Jesus answered, 'I told you that I am he; so, if you seek me, let these men go.' This was to fulfill the word which he had spoken, 'Of those whom thou gavest me I lost not one.'"
It's amazing what you find when you read the Bible as a whole. You find that it is very Catholic....and for good reason!