Even demons believe yet tremble
The believed, they NEVER had saving faith.
But James was not talking about saving faith but the abuse of the church towards the poor by the church professing the church's faith in God's Providence to the poor after church service just to get out of helping the poor. The church was being mean for why James rebuked them that they should lead by example, otherwise the church's faith in God's Providence uttered to the poor will not "profit" nor "save" the poor when the poor sees the church's faith in His Providence as dead by the church not leading by example.
Why do you keep ignoring the words of John? He says they deny christ. That is who they are, and what they have done, they have departed from us, and now have denied Christ.
But that is not the whole truth because John did not address their fate for when they die. Paul addressed that in 1 Corinthians 3:10-17 John singled those going out from us as not abiding in Him for why they are not of us for us to follow them.
When you deny christ, you are apposed to him, you are against him, you deny him. You are an unbeliever.
An unbeliever is not the same as a carnal believer
Titus 1:16 They profess that they know God; but in works they deny him, being abominable, and disobedient, and unto every good work reprobate.
Heresy is a work of the flesh ( Galatians 5:19-21 ) as that is a work that can deny Him. Any work of the flesh on that foundation, defiles the temple of God for why God will deal with it in finishing His work in them too (Philippians 1:6 ), in burning it off of that foundation so that they may be partakers of His holiness even though left behind as disqualified from attending the Marriage Supper in Heaven for not abiding in Him & is words ( John 15:1-8 & Luke 12:40-49 )
A carnal believer is still in faith, he is just serving self. As in the prodigal son
An unbeliever has denied Christ, His issue is more than just sin, although he will be seen by his sinfulness, because he lives as the world does.
A carnal believer is still saved,’
As you can see how a carnal believer, by his work of iniquity, is denying Him, so He will deny him, but I agree that they are still saved just as a former believer is which is why even they are still called to depart from iniquity in 2 Timothy 2:18-21 as that foundation & seal is sure.
If you consider how there are former believers because of the false science, the evolution theory, what would be your task there? To preach the gospel when they already know it? Or to expose the lie that is the evolution theory so the Lord can recover him from that snare of the devil?
2 Timothy 2:24 And the servant of the Lord must not strive; but be gentle unto all men, apt to teach, patient, 25 In meekness instructing those that oppose themselves;
if God peradventure will give them repentance to the acknowledging of the truth; 26 And that they may recover themselves out of the snare of the devil, who are taken captive by him at his will.
So the former believers that believed the lie that the resurrection had passed already, needs scripture to prove otherwise.
Could they be misled by that false teaching that was taking this reference below out of context?
Matthew 27:52 And the graves were opened; and many bodies of the saints which slept arose, 53 And came out of the graves after his resurrection, and went into the holy city, and appeared unto many.
It is possible but hopefully you can see how a heresy can cause a believer to go astray and why as former believers like carnal believers, they are still saved because of that heresy on that foundation for why they do not believe any more & are denying Him because of it.
So like carnal believers, former believers need correction by the scripture to see their need of Jesus's help in forgiveness & repentance.
A non believer has never been saved
To be precise, an unbeliever that never had believed in Jesus Christ has never been saved.
Jesus still abides in former believers for why they and carnal believers are still called to depart from iniquity before the Bridegroom comes.