[HarleyD]

You left out Perseverance of the Saints:

Chapter XVII- Of the Perseverance of the Saints

I. They, whom God has accepted in His Beloved, effectually called, and sanctified by His Spirit, can neither totally nor finally fall away from the state of grace, but shall certainly persevere therein to the end, and be eternally saved.[1]

II. This perseverance of the saints depends not upon their own free will, but upon the immutability of the decree of election, flowing from the free and unchangeable love of God the Father;[2] upon the efficacy of the merit and intercession of Jesus Christ,[3] the abiding of the Spirit, and of the seed of God within them,[4] and the nature of the covenant of grace:[5] from all which arises also the certainty and infallibility thereof.[6]

III. Nevertheless, they may, through the temptations of Satan and of the world, the prevalency of corruption remaining in them, and the neglect of the means of their preservation, fall into grievous sins;[7] and, for a time, continue therein:[8] whereby they incur God's displeasure,[9] and grieve His Holy Spirit,[10] come to be deprived of some measure of their graces and comforts,[11] have their hearts hardened,[12] and their consciences wounded;[13] hurt and scandalize others,[14] and bring temporal judgments upon themselves.[15]

Westminster Confession of Faith (scripture references at site)

Believers can never lose our salvation, yet we should never be so bold as to believe that we could not fall into grievous sin such as David did with Bathsheba which would bring reproach upon our loving God and Savior Jesus Christ. (Yes, David rightfully stated his sin was against God. 2 Sam 12:13) It is to this end we must diligently seeks God's grace and mercy so we are not led into temptation.

Actually, the Apostle Paul claimed those to whom he wrote were Christians in the introduction to both epistles.

Of course he does. He is writing to the church at Corinth. If your priest were to go into the church wouldn't he address everyone as a Catholic? You don't start singling out people. But if you know that the church is holding all sort of drunken orgies and people are not living a Christ-like life, I would hope the priest would have a few choice words to say to them. It is not the responsibility of the priest to tell people they're not a believer. But it is his responsibility to tell them they're acting like pagans (and some of them might be). One can certainly tell them they REALLY, REALLY need to examine themselves if they are in the faith but we can't tell them they're not saved (although we might have our doubts).

Paul, an apostle of Jesus Christ by the will of God

Do you notice anything here about the will of God???

[af_vet_1981]

You left out Perseverance of the Saints:

In post 118 posted on 9/4/2017, 6:59:41 AM by HarleyD the question reads: "Is this why Catholics believe one can lose their salvation?"

The Westminster Confession of Faith is a relatively modern (1646) product of King Henry VIII's Church of England. It is illogical to expect a Catholic to anticipate and rely on it to answer the original question.

This is what the Catholic Catechism contains about perseverance:
1. Perseverance in faith
   
   162 Faith is an entirely free gift that God makes to man. We can lose this priceless gift, as St. Paul indicated to St. Timothy: "Wage the good warfare, holding faith and a good conscience. By rejecting conscience, certain persons have made shipwreck of their faith."44 To live, grow and persevere in the faith until the end we must nourish it with the word of God; we must beg the Lord to increase our faith;45 it must be "working through charity," abounding in hope, and rooted in the faith of the Church.46
   
   
2. 2016 The children of our holy mother the Church rightly hope for the grace of final perseverance and the recompense of God their Father for the good works accomplished with his grace in communion with Jesus.70 Keeping the same rule of life, believers share the "blessed hope" of those whom the divine mercy gathers into the "holy city, the new Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband."71"
   
   
3. Finally, our battle has to confront what we experience as failure in prayer: discouragement during periods of dryness; sadness that, because we have "great possessions,"15 we have not given all to the Lord; disappointment over not being heard according to our own will; wounded pride, stiffened by the indignity that is ours as sinners; our resistance to the idea that prayer is a free and unmerited gift; and so forth. The conclusion is always the same: what good does it do to pray? To overcome these obstacles, we must battle to gain humility, trust, and perseverance.
   
   
4. IV. PERSERVERING IN LOVE
   
   2742 "Pray constantly . . . always and for everything giving thanks in the name of our Lord Jesus Christ to God the Father."33 St. Paul adds, "Pray at all times in the Spirit, with all prayer and supplication. To that end keep alert with all perseverance making supplication for all the saints."34 For "we have not been commanded to work, to keep watch and to fast constantly, but it has been laid down that we are to pray without ceasing."35 This tireless fervor can come only from love. Against our dullness and laziness, the battle of prayer is that of humble, trusting, and persevering love. This love opens our hearts to three enlightening and life-giving facts of faith about prayer.

[af_vet_1981]

Do you notice anything here about the will of God?

Yes, King Saul. Perhaps the Apostle Saul had his namesake in mind when he wrote this to the Corinthians:

But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway.

First Corinthians, Catholic chapter nine, Protestant verse twenty seven,
as authorized, but not authored, by King James