Just an aside, because I think Matt is doing a fine job of answering ...
I am often reminded of Hebrews 6 ...
4For in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit,
5and have tasted the good word of God and the powers of the age to come,
6and
thenhave fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame.
I see no other way to take this than someone who was already "saved" (in Protestant understanding) ... or converted.
They are enlightened. The Holy Spirit must be our Helper in understanding the things of God, they are foolishness to those who are perishing and without the Spirit of Truth.
They have tasted of the heavenly gift. I'm not positive, but I think almost certainly they are in Eucharistic fellowship, which means they have been taught, have been baptized, and have been participating in the believing community and receiving Holy Communion.
They have been made partakers of the Holy Spirit, so again they have been sacramentally brought into the Church (which means they have been converted).
And so on. Some argue that these were "never saved" but that can't make sense. Christians were facing death for converting, sometimes, so how does it make sense that someone would pretend to convert and put their lives as risk for something they didn't believe in at all? I see no other way of understanding this in honesty except to say that it is a warning against falling away from true faith, so if this writing is true and inspired by God (and I think we all agree that it is), then such a thing must be possible.
Hebrews 10 seems to make the same warning, but the 6th chapter is a bit more to the point, I think.
But also, again the point is made that repentance is needed (so it would be accepted by God) ... the problem is the condition of some who might refuse ever to repent.
Lord have mercy.