The Johannine "in me" formula is pretty much
synonymous with Paul's concept of a Christian disciple
being "in Christ." In his high-priestly prayer, Jesus
beseeched his Father in this way (according to the
"I do not ask on behalf of these alone, but for those
also who believe in Me through their word; that they
may all be one; even as You, Father, are in Me and I
in You, that they also may be in Us, so that the world
may believe that You sent Me. The glory which You have
given Me I have given to them, that they may be one,
just as We are one; I in them and You in Me, that they
may be perfected in unity, so that the world may know
that You sent Me, and loved them, even as You have
loved Me" (Jn 17:20-23).
The language above reminds us of the EN verses that we
find in the Johannine epistles (1 Jn 3:5-6, 23; 4:4).
And we must not forget how Jesus expresses himself at
Jn 6:56, a very controversial passage. At any rate, I
see no need to radically differentiate these passages
from those that we find in Paul (see the BDAG entry for EN).
Jesus' statements about being "in" him were proleptic.
That is, they would come to fruition after his death and resurrection.
Jn 15:6 would primarily apply to Christ's disciples
subsequent to the death and Ascension of the Son.
While being "in Christ" chiefly entails
being sealed by God's spirit--a point I don't think John would have
denied--the Scriptures do not teach that the sealing is
necessarily permanent. To the contrary, the sealing mentioned in
the NT is called an ARRABWN, a down payment or earnest
money (Eph 1:13-14; 4:30; 2 Cor 1:21-22; 5:1-5). A Christian must not
only be sealed initially; he or she must preserve the
divine seal until the day of redemption from the
sinful and fallen body (Rev 7:1-8).