"And now I ask you, dear lady - not as though I were writing you a new commandment, but the one we have had from the beginning - that we love one another. And this is love, that we walk according to his commandments; this is the commandment, just as you have heard from the beginning, so that you should walk in it. For many deceivers have gone out into the world, those who do not confess the coming of Jesus Christ in the flesh. Such a one is the deceiver and the antichrist. Watch yourselves, so that you may not lose what we have worked for, but may win a full reward. Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son." - 2 John 1:5-9 ESV
Notice how John warns against losing what we have worked for and not what Christ has done. All of this is said in the context of teachers and teaching. We can't lose what Christ has done but we can lose a very precious possession.
I am convinced that the full reward is a grace understanding centered on the the deity and incarnation of Christ. Anyone who undermines the incarnation is seeking to undermine the authority of grace. The word full (used in verse 8) is the Greek word Pleres (Strong's G4134). It speaks of a container that is full to the brim. That fullness displaces everything else.
Grace must displace everything else or it is not grace. God has either given it or we have earned it. There is no middle ground. If God has given it then we can't lose it. It isn't based on us in the first place.
The incarnation makes the gift possible.
Sleeping beauty stays asleep with no Messiah to wake her up. People find that fairy tale to be dated and sexist because it makes one person reliant on another but it is a perfect parallel for us. We need Messiah and if we forget Him, dilute His work or push him to the side God ceases to be 'in' what we are participating in.
I saw a couple of 'so called' christian movies this Christmas season. They mentioned god and spoke about compassion, forgiveness, reconciliation and redemption but guess who was missing? All those things are wonderful things but they are just not possible without Messiah. True forgiveness without Jesus is a fairy tale because true forgiveness needs the cross.
"Let us therefore strive to enter that rest, so that no one may fall by the same sort of disobedience. For the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and of spirit, of joints and of marrow, and discerning the thoughts and intentions of the heart." - Hebrews 4:11-12 ESV
"But you, beloved, building yourselves up in your most holy faith and praying in the Holy Spirit, keep yourselves in the love of God, waiting for the mercy of our Lord Jesus Christ that leads to eternal life. And have mercy on those who doubt; save others by snatching them out of the fire; to others show mercy with fear, hating even the garment stained by the flesh." - Jude 1:20-23 ESV
Yes. Don't let anyone mess with the grace framework centered on Christ Himself. Guard it. It is what will build up and reach those who doubt. It is our doorway into rest.