Bingo! Hence St. Paul's admonition that we must persevere to the end.
At the end of the day, as a Calvinist, I believe that God must give us his grace if we are to endure to the end. Still, I think the Catholics are right to point out that it would be presumptuous to assume any of us are automatically going to make it.
Double Bingo - Kudos, my friend, for your astute observation. God gives us the graces but He also gives us free will. In Matt. 12:30-32, Jesus says that he who is not with Him is against Him, therefore (the Greek for "therefore" is "dia toutos" which means "through this") blasphemy against the Spirit will not be forgiven. This means that failing to persevere in Jesus' grace to the end is the unforgivable sin against the Spirit. We must persevere in faith to the end of our lives. John 6:70-71 reminds us that Jesus chose or elected twelve, yet one of them, Judas, fell. Not all those predestined to grace persevere to the end.
That's where you and I part ways. Rom. 8 couldn't be more clear that the predestined are all justified, sanctified, and glorified.
My point is that I don't know if I'm one of those chosen to persevere to the end. I sure think so, and hope so, and intend to stick it out - but if I were to fall away (irrevocably) it would be because I'm one of the self-deluded whom Christ does not know.