I am convinced of Sola Fide after strong verses like John 3 16. Works based righteousness to me has always been a terrible idea that culminated in indulgences, and nowadays in going to a church you don't particularly care for or like, just to be saved. It sucks to sit through a bad sermon normally, but with the "I HAVE to do this or I will go to Hell" mentality, this gets terrible. Even the priests can tell who is going there just to be a "practicing" Catholic and what not.
My source of confusion is in who doesn't get saved. Things such as abortions or miscarriages make this even harder, since it's hard to call someone who didn't live a sinner. This creates a loophole where, in the theology of some denominations, you can microwave a baby, get baptized, and you will enjoy Heaven while the baby sits in Hell for eternity
When you recognize God's sovereignty and that He elects His saints, then there is no point in questioning the non-elect. Though it still lingers that some non-elect are easier to sympathize with than others.
On who is part of the elect, then I would probably draw the line liberally. If you are a church-going Christian who admits Christ is King, I have a hard time believing you can turn back to an atheist for the rest of your life.
If one denies Christ at some point after being Saved then it's feasible that they never truly accepted him in the first place.
This is a good argument for Perseverance of the Saints. I'd say that in accepting Christ you have two points, which are believing He existed, and believing He was the Son of God.
His existence is well accepted, but the tricky part is faith in Him. Nominal Christians have an issue with this. Either in applying liberalism to His Word, missing the point and turning to asceticism within a Church, or just falling for a false movement in general. Some stumbling blocks are bigger than others, but denying Christ and His teachings is the worst one.