You are correct osas is a doctrine of demons! Dig deep, seek His truth, endure to the end, or else your faith can become shipwrecked!
And in your digging and seeking, don't overlook:
Jn 10:27-29 - "My sheep listen to my voice. I know them and they follow me. I give them eternal (everlasting) life, and
they shall never perish; no one can snatch them (including they themselves cannot escape) out of my hand. My father who has given them to me is greater than all; no one can snatch them out of my Father's hand."
Jn 6:39 - "And this is the will of him who sent me, that
I shall lose none of all that he has given me, but raise him up at the last day. For my Father's will is that eveyone who looks to the Son, and believes in him (true faith, not false faith) shall have eternal life."
Php 1:6 - ". . .he who began a good work in you
will carry it on to completion until the day of Christ Jesus (second coming)."
Heb 10:14 - ". . .he has
made (past tense) perfect forever those who are being made holy (the saved)."
Ro 8:38 - ". . .neither death nor life. . .nor
anything else in all creation, will be able to separate us (the saved) from the love of God in Christ Jesus our Lord."
1Pe 1:3-5 - ". . .he has given us new birth. . .into an inheritance. . .who through faith are
shielded by God's power (from falling away) until the coming of the salvation that is ready to be revealed in the last time (second coming).
2Pe 1:10-11 - ". . .
make your calling and election sure (to yourself). For if you do these things, you will never fall (from faith), and you will receive a rich welcome into the eternal kingdom. (If you are not doing these things, your calling and election are not sure; i.e., you were never called and elected.)
Ro 8:28 - ". . .
in all things God works for (accomplishes) the good of those. . .who have been called according to his purpose (salvation)." That excludes falling from faith.
Eph 1:4-5 - ". . .
he chose us. . .to be holy and blameless. . .
to be adopted as his sons."
Regeneration (born again) does not allow that, once having been redeemed from the rebel camp of Satan and adopted into God's family as sons,
we can then be snatched from God's hand (contrary to Jn 10:27-29). That makes God too small.
Consider
what regeneration really is:
Regeneration is raising from spiritual death (Col 2:13; Eph 2:1, 5) and the old nature (Eph 2:3),
into eternal life (God's life--2Pe 1:4; 1Jn 3:9; Jn 14:17), and a new nature/self (Eph 4:24; Col 3:10),
which makes us new creatures (2Co 5:17), like the transformation process of a caterpillar into a butterfly (Ro 12:2; cf Mt 7:2; Mk 9:2).
To "lose salvation" is to be snatched from the Father's hand, to reverse all that mighty work of God, and to reverse the transformation process, like the butterfly going back into a caterpillar.
That is imposible in the natural order, and it is impossible in the spiritual order, of which the natural order is a copy,
as the natural order is a copy of the spiritual order in conception and birth, where we have no choice in the matter (Jn 1:13), and are completely acted upon.
Scripture often speaks of
professors (who are not possessors) as in the kingdom, but not of the kingdom.
See Lk 8:13-15; 1Jn 2:19; Mt 13 parables; Jn 8:30-31, 44, 15:2, 6; Gal 5:4; Heb 6:4-6, 10:29.
The specific phrase, "in the kingdom, but not of the kingdom," is not stated in Scripture, but specific illustrations of it are given; e.g.:
Lk 8:13-15, where the word is received with joy when they hear it, but they have no root (regeneration).
They believe for a while (false faith), but in the time of testing they fall away (because they have no root, they are not reborn).
They were
in the kingdom for a while, but they were not
of the kingdom.
1Jn 2:19 - "They went out from us (the kingdom), but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going was to show that none of them belonged to us (the kingdom).
They were in the kingdom, but they were not of the kingdom.
In the parables in
Mt 13 on the nature of the kingdom, those in 13:24-30 are weeds
in the kingdom (v.25, 41), but not
of the kingdom (v.30, 41).
Likewise, those in 13:47-50 are bad fish in the net (kingdom), who are in the kingdom (net-v.47), but not of the kingdom (vv.48-49).
In
Jn 8:31-47, those who believed in Jesus (v.31) did not hold to his teaching (v.31-33).
They were in the kingdom, but not of the kingdom (vv.44, 47).
They belonged to the enemy (Jn 8:44), not to the kingdom,
just as in the parable of the the weeds, they belonged to the enemy (Mt 13:25), not to the kingdom.
In
Jn 10:1, the robber who climed into the sheep pen (kingdom of God), rather than entering through the gate (Jesus), was
in the kingdom, but not
of the kingdom. He belonged to the enemy, not to the kingdom, as in Mt 13:25 and Jn 8:44.
In
Jn 15:1, Jesus is the vine (kingdom) where the Father cuts off the branches that bear no fruit. Those branches are in the kingdom, but not of the kingdom.
In
Gal 5:4, they have fallen away from grace; i.e., they have gone to law-keeping. Those who continued in law-keeping after being warned of the consequences were in the kingdom, but not of the kingdom. Those of the kingdom abandoned law-keeping.
Heb 6:4-6, 10:29 is the same kind of situation. Those who returned to Judaism after being warned of the consequences were in the kingdom, but not of the kingdom. Those of the kingdom did not return to Judaism.
So while the phrase "in the kingdom but not of the kingom" is not specifically stated, it is specifically illustrated in the above (as the phrase "sovereignty of God" is not specifically stated, but it is specifically illustrated--Da 4:35).
Those who fall away or leave the fold were never really saved in the first place.
The NT is clear that the born again do not lose their salvation.