At the very least, most Christians believe that God has always known whether or not a man would be saved. Or else He's not omniscienct. He's not God.
If, at the moment of creation, God has "known" a person's destiny, is that "destiny" not fixed in stone?
Scripture tells us that God has called a particular people to Himself. And that people He will save. We have His promise in Scripture that He will give us the Holy Spirit to lead our education and guide our sanctification as we move ever closer to Him. That sanctification includes understanding truth from error.
Do men understand all truth? No. All men are fallen and as Paul says, we sin every day. But we are being refined, according to His purpose in creating us, which is to glorify Him in thought, word and deed.
So we can be reasonably certain that God is aligning His children according to the truth of His word. From my perspective, that does not include veneration of Mary as co-redeemer or following "another Christ" or thinking that sins are perpetually committed and forgiven over and over in an endless sacrifice for sins when Hebrews tells us clearly that Christ's one-time sacrifice on behalf of His sheep has been accepted by God as sufficient to repay the debt, and that God graciously remembers our sins no more.
Are Roman Catholics saved? Some probably are.
Calvin said "Although the devil has long reigned in the papacy, yet he could not altogether extinguish God's grace: nay, a Church is among them."
I could agree with that.
That wasn't the question.
Does it matter, can it possibly matter in one's salvation whether one is a full-bore believing Catholic or not? According to predestination - no.
Or else He's not omniscienct.
Omniscience isn't the point either. It is Calvin's god's double predestination: that some are doomed in the womb and some are saved in the womb. And nothing can change that.
Exhorting people to join a "bible believing" church can have no effect on their salvation either - whether they do or do not. Else double predestination is false.
If double predestination is true, your posts here, not to mention Jesus's entire ministry are only exercises in futility.