John 15 has several references to loss of salvation.
John 15:2 (KJV) Every branch in Me that does not bear fruit He takes away;fn and every branch that bears fruit He prunes, that it may bear more fruit.
The agricultural metaphor was used to indicate production, or lack of production. Just as a farmer will discard a branch that does not produce fruit, So God will discard the believer (not use for His service) who doesn't produce.
One must remember how God relates various doctrines. He is called our Father, and we are called His children.
Just as the physical RELATIONSHIP between parent and child cannot be undone, or severed, the same is true of the spiriutal RELATIONSHIP between God and His children.
What can be broken or severed is FELLOWSHIP between God and His children.
Here are some veres about FELLOWSHIP:
1 Cor 1:9 - God is faithful, through whom you were called into fellowship with His Son, Jesus Christ our Lord.
2 Cor 13:14 - The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit, be with you all.
Phil 2:1 - Therefore if there is any encouragement in Christ, if there is any consolation of love, if there is any fellowship of the Spirit, if any affection and compassion
1 John 1:3 - what we have seen and heard we proclaim to you also, so that you too may have fellowship with us; and indeed our fellowship is with the Father, and with His Son Jesus Christ.
1 John 1:6 - If we say that we have fellowship with Him and
yet walk in the darkness, we lie and do not practice the truth
1 John 1:7 - but if we walk in the Light as He Himself is in the Light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin.
(NASB)
Just as fellowship between birth parent and child can be lost, broken, severed, etc, the same is true of our fellowship with God.
In fact, Paul describes loss of fellowship this way:
Eph 4:30 - Do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption. NASB
1 Thess 5:19 - Do not quench the Spirit NASB
It should be obvious that either grieving or quenching the Holy Spirit would be a break in FELLOWSHIP. Just as grieving our birth parents illustrates a break in fellowship.
How can a child have fellowship with the parent they are grieving? Not possible.
FELLOWSHIP must be restored between God and His children when it has been broken. Just like the prodigal son, who repented and confessed his sins to his father.
That's why we have 1 Jn 1:9. To restore fellowship.
We must know and remember that fellowship and relationship are not the same. Unfortunately, most people today use the word "relationship" when they really mean fellowship.
The understanding of the word translated "takes away" (Gr: αἴρω [airō] to move from its place, to take off or away what is attached to anything, to remove), is reflected in the word translated "prunes." ( Gr: καθαίρω [kathairō] to prune trees and vines from useless shoots). A branch that is "taken away" from the vine is no longer connected to the vine. Both words indicate that unproductive branches are removed (taken away, removed, cut off, pruned) from the vine.
And this speaks of loss of fellowship. The reason this CANNOT refer to loss of salvation is that the believer's RELATIONSHIP cannot be broken any more than the RELATIONSHIP between birth parent and child cannot be broken.
Further, Paul taught that God's gifts are irrevocable in Rom 11:29. And he described eternal life as a gift of God in Rom 6:23. And he never used the word 'gift' between those 2 words. So they are definitely connected.
When Paul wrote 11:29, he most certainly was thinking of eternal life, which he described as a gift of God.
Jesus reiterated the teaching at verse 6: “If anyone does not abide in Me, he is cast out as a branch and is withered; and they gather them and throw them into the fire, and they are burned." (NKJV)
Both "fire" and "burning" are metaphors usually used to denote God's judgment or discipline. When indicating the lake of fire, we find added adjectives, like "eternal fire" (Matt 18:8, 25:41, Jude 7).
That verse states the fact that, according to Jesus, it is possible to be "in Christ" and not remain (abide) in Him.
Correct!! To be 'in Christ' is a permanent RELATIONSHIP. We know this from several verses:
Paul spoke of believers being sealed with the Holy Spirit:
13In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise,
14who is given as a pledge of our inheritance, with a view to the redemption of
God’s own possession, to the praise of His glory. Eph 1 (NIV)
Do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption. Eph 4:30 NIV
who also sealed us and gave
us the Spirit in our hearts as a pledge. 2 Cor 1:22 NIV
Now He who prepared us for this very purpose is God, who gave to us the Spirit as a pledge. 2 Cor 5:5 NIV
The Holy Spirit is a PROMISE, with a view to the REDEMPTION of God's own possession (believers), and sealed FOR THE DAY OF REDEMPTION.
It could not be said more clearly that this.
Once sealed, always sealed. For the day of redemption.
One cannot be saved if one does not remain in Christ.
But you just noted that one can be "in Christ" yet not "abide in Christ". This describes the difference between RELATIONSHIP and FELLOWSHIP.
To be "in Christ' speaks of RELATIONSHIP. Sealed with the Holy Spirit for the day of redemption.
To "abide in Christ" speaks of FELLOWSHIP. Which can be broken.