Catholics do believe that The Elect persevere–but only God knows who The Elect are!
Here’s the problem, though: that is
precisely the Protestant (OSAS) understanding of eternal security. The implication of that belief is that, if you are
truly saved (i.e., at your baptism), then nothing you can do will tear you away from God, but that, if you’re
not truly saved, then you’re ripe for the plucking.
The danger of this doctrine is the implication that, if we say “I’m a true Christian” then we can act with impunity, because we’re implicitly saying that we’re already saved and nothing we do can change that.
Catholics also believe that those who are once saved or currently saved do not necessarily always equal those who will be saved–The Elect–in other words from a human viewpoint which is in time and not outside of time like God’s viewpoint is–one can lose his salvation.
Again, this is not substantially different than the OSAS belief; the only difference between the two are the labels you use. What you, here, call “will be saved”, they call “already saved”, and what you call “once saved” or “currently saved” – with the implication that they lose their salvation – they call “unsaved”.
The place that the Catholic and OSAS doctrines diverge is that the OSAS side says that, among the living, there are already those who are ‘saved’; the Catholic doctrine says that we cannot place the label ‘saved’ on those who are still alive. We who are alive are still “working out our salvation”; we are in the process of being saved (having been justified and having been cooperating with the graces we received at baptism and through the reception of the sacraments). As Catholics, we say that the label ‘saved’ can only apply to us once we have passed away, and at the particular judgment Jesus has told us that we have inherited heaven.
While Catholics can never presume that they will persevere with a 100% certainty they can have a moral certainty–which is less than absolute certainty–that IF in God’s view that they obey God’s commandments–are in a state of grace–receive the sacraments–and do “for the least of Jesus’ brethren”–that they will be saved.
Right – we have security, but only in the faithfulness of God to His promises.