The POTENTIAL for them not to abide is there in the 2nd to the last verse of the passage:
"28Now, little children, abide in Him, so that when He appears, we may have confidence and not shrink away from Him in shame at His coming."-1 John 2:28
Why say such a thing if it's a given that they can't not abide (continue) in him?
Little children/believers abide in Him, have confidence and will not be ashamed before Him at His coming. In CONTRAST, there will be those shallow, temporary believers whose faith was never firmly rooted and established from the start and did not abide (like those in the 2nd soil) and face only shame at His coming. To abide simply means to not to depart, to continue to be present, to be held, kept, continually.
Paul plainly said in 1 Corinthians 15:1-2 that they believe and are saved, but that this genuine believing is in vain if Christ really did not rise from the dead.
No, Paul did not say that genuine believing is in vain. Paul makes a CONTRAST between receiving the gospel/in which you stand/are saved (demonstrative evidence)
IF you hold fast that word which he preached with
UNLESS you
believed in vain -- without cause or without effect, to no purpose. This contrast is found in verses 1 and 2.
"1Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, 2by which also you are saved (present tense), if you hold fast the word which I preached to you, unless you believed in vain."-1 Corinthians 15:1-2
Either we believe and hold fast or else we believe in vain. There is no middle ground.
In Matthew 6:7, we read - And when you pray, do not use
vain repetitions as the heathen do. For they think that they will be heard for their many words.
In Matthew 15:9, we read - And in
vain they worship Me, Teaching as doctrines the commandments of men.
By 1 Corinthians 15:12-19, Paul has already moved on from the point he made in verse 1-2 and is making a new point. 12 But if it is preached that Christ has been raised from the dead,
how can some of you say that there is no resurrection of the dead? 13
If there is no resurrection of the dead, then not even Christ has been raised. 14 And
if Christ has not been raised, our preaching is useless and so is your faith. 15 More than that, we are then found to be false witnesses about God, for we have testified about God that he raised Christ from the dead. But he did not raise him if in fact the dead are not raised. 16
For if the dead are not raised, then Christ has not been raised either. 17 And
if Christ has not been raised, your faith is futile; you are still in your sins. 18 Then those also who have fallen asleep in Christ are lost. 19 If only for this life we have hope in Christ, we are of all people most to be pitied.
So there is no argument here that somehow they are not really saved. None whatsoever.
The one's who
fail to hold fast to the gospel and
believed in vain --
without cause or without effect, to no purpose are not really saved.
To compare the believing of the 2nd soil to that of demons is not even reasonable. No demon believes falsely for a while with joy. No demon has the word growing in his heart if even only shallowly.
Yet the point that I was making is that NOT ALL belief is saving belief. In James 2:19, nobody is questioning the fact that the demons believe (pisteuo) "mental assent" that "there is one God" but they
do not believe/entrust their spiritual well being to Christ; have faith/reliance upon Christ for salvation. In other words, they
do not believe (same Greek word - pisteuo)
on the Lord Jesus Christ (Acts 16:31) and are not saved. Their trust and reliance is in Satan, as demonstrated by their rebellion in heaven and continuous evil works.
Again, people who hear and receive the word with joy (emotional response) and believe (in a shallow way)
without a good and honest heart, and without having "root" do not experience real salvation.
There is a stage in the progress of belief in Jesus that "falls short of genuine or consummated belief resulting in salvation." As we see with this superficial belief in John 2:23-25 and in John 8:30, where these Jews who were said to have "believed in Him" turn out to be slaves to sin, indifferent to the words of Jesus, children of the devil, liars, and guilty of setting out to stone the one they have professed to believe in (verses 34-59).
I suppose you think the little ones who believe in Jesus in Matthew 18:6-9 are fake, demonic believers too? You know the little ones who believe in Jesus who if caused to stumble and go to the eternal fire will condemn the person who caused them to stumble to a fate worse than drowning in the sea.
Not everyone is a fake/make believer and who said that "stumble" equates to go to the eternal fire/condemnation? In Matthew 26:31 - Then Jesus said to them, "All of you will be made to
stumble because of Me this night, for it is written: 'I will strike the Shepherd, And the sheep of the flock will be scattered.' Does that mean all of Jesus' disciples are condemned to eternal fire? Are all 12 disciples condemned?