If the ones who leave were never saved to begin with, why does John warn the remaining believers, who he says are saved and know the truth, to let the truth abide in them so that they will abide in the Son and the Father?
"As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide (continue) in the Son and in the Father."-1 John 2:24
1 John 2:24 -Therefore let that abide in you which you heard from the beginning.
If what you heard from the beginning abides in you, you also will abide in the Son and in the Father. 25 And this is the promise that He has promised us--eternal life. 26 These things I have written to you concerning those who try to deceive you. 27
But the anointing which you have received from Him abides in you, and you do not need that anyone teach you; but as the same anointing teaches you concerning all things, and is true, and is not a lie, and just as it has taught you,
you will abide in Him.
See, the word has to continue to abide in you for you to continue to be saved. Paul echos this same truth in this verse:
If what you heard from the beginning abides in you, you also will abide in the Son and in the Father. Nothing there about some will continue to abide and some will not.
"you are saved, if you hold fast the word which I preached to you, unless you believed in vain (if Christ has not really risen from the dead your faith is in vain)."-1 Corinthians 15:2
1 Corinthians 15:1,2 - Moreover, brethren, I declare to you the
gospel which I preached to you, which also you received and in which you stand, by which also you are saved,
if you hold fast that word which I preached to you--unless you believed in vain. *To believe in vain is to believe
without cause or without effect, to no purpose. If, as some are saying in Corinth,
there is no resurrection, then faith is vain and worthless (vs. 14). The people who fail to hold fast to the word (the gospel) that Paul preached in 1 Corinthians 15:1-4, demonstrated that they
"believed in vain" (did not truly believe in the first place).
The 2nd kind of soil believer does not retain the word. They believe, but the word is not deeply rooted in them. When trouble or temptation comes they fall away and do not continue in the Son and the Father. They will not be saved when Jesus comes back.
Shallow, temporary belief that has no root, produces no fruit and withers away is not saving belief. The demons "believe" (James 2:19) but they are not saved. In John 8:30, these Jews who were said to have "believed in Him" turned out to be slaves to sin (verse 34), indifferent to the words of Jesus (verse 37), children of the devil (verse 44), liars (verse 55), and guilty of setting out to stone the one they have professed to believe in (verse 59). The "belief" in John 2:23-25 is superficial in nature. So as we can see, NOT ALL belief is the same.