Which gospel?

Your assessment is half correct. Yes, Paul is writing to them convincing them of the resurrection of Christ because at that time they had been swayed to believe that Christ had not risen, but this doesn't change the fact that Paul had already preached to them earlier about the risen Christ and doesn't change the fact that they had believed the true gospel which makes Paul's statement (vs1-3) clearer - They did not hold fast to what they were taught earlier.

Neither here nor there in the discussion. The larger point Paul was making is that if Christ didn't rise from the grave, THEN they have believed in vain.

They might as well party like it's 1999, because THEY would have no resurrection either, and their belief in Christ as the atoning Sacrifice is in vain without the Resurrection.

How?!

When the bible talks of apostasy, how can a non believer apostatize? Please explain (20marks)

They are basically the second soil. They heard the Gospel, but didn't truly believe, because they had NO Root. JESUS is the Root.

So they had a false profession, but fell away. But NONE of them were converted Brethren.

The fruit determines what is to be done with the tree.

So why is Paul telling people to hold fast if it is the heavenly Father that works?

John 15:
4Remain in me, as I also remain in you. No branch can bear fruit by itself; it must remain in the vine. Neither can you bear fruit unless you remain in me.

5“I am the vine; you are the branches. If you remain in me and I in you, you will bear much fruit; apart from me you can do nothing. 6If you do not remain in me, you are like a branch that is thrown away and withers; such branches are picked up, thrown into the fire and burned. 7If you remain in me and my words remain in you, ask whatever you wish, and it will be done for you. 8This is to my Father’s glory, that you bear much fruit, showing yourselves to be my disciples.

9“As the Father has loved me, so have I loved you. Now remain in my love. 10If you keep my commands, you will remain in my love, just as I have kept my Father’s commands and remain in his love.

But, IMO, the largest point Paul made is:

...By this gospel you are saved, if you hold firmly to the word I preached

and what did Paul NOT teach?

gentiles keeping the Sabbath as a part of being a Christ follower.

because it is not a requirement, as you judeaizers lie and say it is.

This defeats your own position. If they did not truly believe, then they are still unbelievers/ satan's children/ still not of the faith/ not in grace

And so it can never be said that they have departed from faith or fallen from grace and Paul in his right mind can never tell one of them that they have become worse than unbeliever like he did here:

1 Tim 5:8Anyone who does not provide for their relatives, and especially for their own household, has denied the faith and is worse than an unbeliever.

What is there to misunderstand in those two very plain and clear verses?

For by grace -- SALVATION IS PURELY BY GOD'S GRACE

are ye saved -- SALVATION IS A FINISHED WORK OF GOD

through faith -- FAITH IN CHRIST AND HIS FINISHED WORK IS NECESSARY

and that not of yourselves -- NO ONE CAN SAVE THEMSELVES

it is the gift of God -- ETERNAL LIFE IS THE GIFT OF GOD

Not of works -- NO WORKS CAN SAVE ANYONE

lest any man should boast -- BOASTING IS THEREFORE EXCLUDED

For we are his workmanship -- BELIEVERS ARE GOD'S WORKMANSHIP

created in Christ Jesus -- BELIEVERS ARE NEW CREATURES IN CHRIST

unto good works -- GOOD WORKS ARE NECESSARY FOR THE SAINTS

which God hath before ordained -- GOOD WORKS ARE DIVINELY ORDAINED

that we should walk in them -- THE CHRISTIAN WALK INCLUDES GOOD WORKS

Good works are not done in order to MAINTAIN salvation but in order to WORK OUT your salvation. This is the true Bible doctrine.

Yep. More description of what the born again Child of God WILL do.

HE, is the One who prunes.

You STILL think it's about the fruit when it comes to Salvation!

Matthew 7:18 New King James Version (NKJV)
18 A good tree CANNOT bear bad fruit, nor can a bad tree bear good fruit.

Nope. That's YOUR understanding because you cannot get out of a works, Jesus is too weak to keep you saved mentality.

Unfortunately, I can explain it to you, but I can't comprehend it for you. I mostly stay engaged with the like of you for outside readers who may become confused and deceived by a false works gospel.

Well of course that's your opinion. You don't trust Christ to save, and keep you saved.
No mystery there.

Sabath was not about the day but it was about love for others. God made it clear:

Exo 20:8Remember the Sabbath day by keeping it holy. 9Six days you shall labor and do all your work, 10but the seventh day is a Sabbath to the LORD your God, on which you must not do any work—neither you, nor your son or daughter, nor your manservant or maidservant or livestock, nor the foreigner within your gates.

Isa 58:
1“Cry aloud, do not hold back!
Raise your voice like a ram’s horn.
Declare to My people their transgression
and to the house of Jacob their sins.

2For day after day they seek Me
and delight to know My ways,
like a nation that does what is right
and does not forsake the justice of their God.
They ask Me for righteous judgments;
they delight in the nearness of God.”

3“Why have we fasted, and You have not seen?
Why have we humbled ourselves, and You have not noticed?”
“Behold, on the day of your fast, you do as you please,
and you oppress all your workers.

4You fast with contention and strife
to strike viciously with your fist.
You cannot fast as you do today
and have your voice be heard on high.

5Is this the fast I have chosen,
a day for a man to deny himself,
to bow his head like a reed,
and to spread out sackcloth and ashes?
Will you call this a fast
and a day acceptable to the LORD?

6Isn’t this the fast that I have chosen:
to break the chains of wickedness,
to untie the cords of the yoke,
to set the oppressed free
and tear off every yoke?

7Isn’t it to share your bread with the hungry,
to bring the poor and homeless into your home,
to clothe the naked when you see him,
and not to turn away
from your own flesh and blood?

So, are we to keep sabath? Yes, just love others.
Did Paul teach sabath keeping? Yes, 1 Cor 13

You have a big list but you miss one thing, the grace of God is for all mankind/ sufficient:

Titus 2: 11For the grace of God has appeared, bringing salvation to all men.

Titus 3:4But when the kindness of God our Savior and His love for mankind appeared, 5He saved us, not by the righteous deeds we had done, but according to His mercy, through the washing of new birth and renewal by the Holy Spirit

Rom 5:18So then, just as one trespass brought condemnation for all men, so also one act of righteousness brought justification and life for all men.

Col 1:20and through Him to reconcile to Himself all things, whether things on earth or things in heaven, by making peace through the blood of His cross.

Q. If grace has saved all men, why will some be judged as not saved?

Nope. Jesus is not describing the brunches that remain in Him, He is telling them what to do to remain in Him:

John 15:
7If you remain in me and my words remain in you, ask whatever you wish, and it will be done for you. 8This is to my Father’s glory, that you bear much fruit, showing yourselves to be my disciples.

9“As the Father has loved me, so have I loved you. Now remain in my love. 10If you keep my commands, you will remain in my love, just as I have kept my Father’s commands and remain in his love. 11I have told you this so that my joy may be in you and that your joy may be complete. 12My command is this: Love each other as I have loved you.

If you keep His commands, you'll remain in Him- means if you don't then you will be cut out. IOW, it is your choice to remain in Him. But let no one cheat you, love is a work.

Nope. Wrong again cadet!

I did NOT miss anything. The grace of God is offered to all mankind but brings salvation only to those who believe on the Lord Jesus Christ. That is the meaning of "by faith".

Those are the ones who are JUSTIFIED by grace through faith, and receive the gift of eternal life. Those are also the ones who received the gift of the Holy Spirit, and are born again by the power of the Spirit, to become new creatures in Christ.

Those who are justified have the righteousness of God and Christ IMPUTED to them. Because they are IN CHRIST God deems them to be as righteous as Christ. Therefore their good works can add nothing to their salvation. But there will be rewards for good works done with the right motive.

I do trust Christ but differently. He is the one who said that whoever perseveres to the end will have their soul saved.

So then its a thousand literal years plus a unknown... in limbo or purgatorial ?

I would offer. The last day. Not last hour or month last day is used 7 times in John to execute his judgment and in the same twinkling of the eye raise those who have been promised a new incorruptible body. They will be nether male or female, Jew or gentile. As sons of God what we will be is clearly not of that which we are as a corrupted flesh and blood creation. .

It is the last day, not month or a literal thousand years. Christ will come as a thief in the night. We can watch like Noah but he will still come in a time no one expects. We walk by faith the unseen. . not after the lusts of the eye.

I would believe we are those who walk by faith the (unseen) And are not of the number as those who number people or days .David was enticed in that way. Many died

The "last day" is the same "latter day" spoken of in Job. Not latter days, the days we are in. . Signaled by the renting of the veil. or it is finished.

Job 19:25 For I know that my redeemer liveth, and that he shall stand at the latter day upon the earth:

If we would followed the signified interpretation instructions in the opening passage of Revelation 1:1. .Then it would seem easy to see when looking at the parable in Revelation 20 from the prospect of the last day the end of time shown in the previous chapter 19..

There would be no need in mixing a literal understanding in regard to time along with the rest of the metaphors that give us the understand of faith, the eternal not . Not literal key, no literal chain, no literal bottomless pit, no literal years, no literal serpent. no literal headless souls as disembodied spirit workers , some call patron saints.

It would be like the parable in Luke with the unbelieving rich man that sought after necromancy like Catholisicim seeing no value in the things of God, as all things written in the law and the prophets, (sola scriptura) the unseen power of the gospel.

The Amil position, the position at the time of reformation seems to work the best. Many have been added .Why?

Revelation 1 King James Version (KJV) The Revelation of Jesus Christ, which God gave unto him, to shew unto his servants things which must shortly come to pass; and he sent and signified it by his angel unto his servant John:

Mmh, let's try explaining how a non believer can depart from faith or apostatize.

You see, there's only two positions with no middle ground. When it comes to Christ, you are either a believer or a non believer. A believer can depart from their position and become a non believer and a non believer to being a believer. So when you say a non believer fell from faith or apostatized and did not become a believer, what do you really mean?

I'll not hold my breath in wait for an answer.