It is to see how the Reformed deal with those who leave Calvinist. It is my belief that either Total Ability (T) or Preservation of the elect (P) in TULIP is compromised.
EmSw, you say "Total Ability". To my knowledge - - according to Calvinist belief, "T" does not mean "Total Ability", but Total Depravity. And "Total Depravity" can be tailored to mean what it does not mean, so people can criticize TULIP people. What I have read in a Calvinist writing did not match with how certain people have represented Calvinists to mean by "Total Depravity".
So, I suppose "Preservation of the elect" can be misunderstood and misrepresented. If we disagree with something, I understand that God wants us to correctly represent what people believe while disagreeing with them.
So, have you given your understanding of "Preservation", along with an actual quote of the official Calvinist representation of it? Or, have you been disagreeing with third-party representations?
Speaking for myself, I do not personally know John Calvin so I could really know what he meant by what he has spoken or written. People writing history and interpretations could be not understanding him correctly, for all I know. So, I see what I get through prayer and the Bible, but knowing I am not perfect at prayer and receiving how God has me understanding things. So, I will simply offer what I have.
I think "total dependence" is more like it >
"Not that we are sufficient of ourselves to think of anything as being from ourselves, but our sufficiency is from God," (2 Corinthians 3:5)
From this, I see that we humans do not have any ability of our own selves to wish or will or do or get anything really right. So, we are totally dependent on God and His grace which is the action of His own love effecting our nature and behavior and lives. No being less than God can produce all that is really right and of real love, and no being less than God is capable of oneself even wishing this, really and honestly. But God in us works what is right, for both willing and doing >
"for it is God who works in you both to will and to do for His good pleasure." (Philippians 2:13)
Even before Adam and Eve fell, while they still were perfect as creatures, it was not their nature to be and stay how God's love is. Only Jesus ever was meant to be the first truly perfect Human on this earth. We all have never had a chance.
This I see from reading the Bible and from how well I have seen humans getting themselves to even wish to be perfect in love; in any case, I have seen my own failure.
Now, is this what Calvinists mean by "T"? I can't speak for people I don't even know personally. But this to me means how I need to constantly and fully rely on God, at every moment submitting to Him . . . but how He in me has me succeeding in this, with no self-propulsion, at all!
About "P"erseverance of the saints >
@EmSw > Em, you seem to say that if a person leaves Calvinism, this means the person has not persevered the way Calvinists believe the elect will persevere. Well, I consider that mainstream Calvinists might understand that a person who doesn't persevere either was not truly saved, in the first place, or the person is elect but change of beliefs, alone, would not mean the person has quit, but only changed in one's ideas. But I'll bet not all Calvinists, themselves, have the same ideas about this

So, I would let each person speak for oneself, and not assume that certain people speak for everyone, only because they claim that all believe what they do.
I understand that if a person truly trusts in Jesus (Ephesians 1:12), this person will persevere. So, if one is not of God, the person could for a while think he or she is, or be pretending on purpose. And if the person becomes a Christian while understanding Calvinist people's teachings, the person could grow to learn more and better which is in the Bible, then leave behind any Calvinist items which do not match with how the person discovers the Bible.
Ones, by the way, can put on quite the drama that they are sincere. If you have ever watched a movie, perhaps you know how an actor can have you experiencing the actor to be someone who that actor is not, at all. So, you being convinced by how someone acts . . . or writes . . . means how much??
The Bible says there are people who have left because they never were children of God, in the first place > 1 John 2:19. But does this mean only changing from certain beliefs of a group? I think it means leaving Jesus Christ after making some indication of being a Christian. It is saying ones who leave God's people were never with Christ, really, in the first place, I consider > 1 John 2:19.
"But he who is joined to the Lord is one spirit with Him." (1 Corinthians 6:17)
If a person is "joined" spiritually to Jesus Himself, I understand the power of this union is almighty so the person can not get away from Jesus. And in this union, the power of the Holy Spirit wins out over how the person's own nature is, so that the person changes to become obedient and therefore incapable of disobeying God to the point of totally getting away from Him. But the person is not perfectly like Jesus, right away. We grow in God's love; but our union with Him Himself got us started, at the time we trusted in Jesus.
So, Biblical persevering of God's people means we keep on getting more of God's correction (Hebrews 12:4-11) of His love's perfection (1 John 4:17). And if we sin, we can get very homesick, very quickly; because sinning degrades us from Jesus Christ's "rest for your souls" (Matthew 11:28-30) which we are experiencing and appreciate so dearly and deeply. So, perseverance includes how we are growing in God's own love with His own Heavenly goodness in us > Romans 5:5. I do not know if John Calvin says anything about this, or if TULIP people do. But I find this in the Bible and my personal experience
