The Church is found throughout Revelation.

Those of us Christians who will be in
the tribulation will be Christians in
no different way than those who will
die before the tribulation -- we will
be from every nation (Rev 7:9), will
have washed our robes in the blood of
the Lamb (Rev 7:14), will have the
testimony of Jesus Christ (Rev 12:17)
and the faith of Jesus (Rev 14:12),
and many of us will be beheaded for
the witness of Jesus (Rev 20:4).

Now, what religion will we be if not
Christian?

After the cross and after Pentecost,
there can be no Christians separate
from the Church. There is no division
in the body of Christ:

Eph 1:22-23
The church, Which is his body...

Eph 4:4-6
There is ONE body, and one Spirit, even
as ye are called in one hope of your
calling; One Lord, one faith, one baptism,
One God and Father of all, who is above
all, and through all, and in you all.

1 Cor 12:25
There should be no schism in the body...

1 Cor 12:13
By one Spirit are we ALL baptized into
one body...

1 Cor 1:13
Is Christ divided?

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Christians in the tribulation are after
Pentecost, so they must have the Spirit:

Rom 8:9
If anyone does not have the Spirit of Christ,
he does not belong to Christ.

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Even believers who died before the cross
or before Pentecost have now been brought
into the Church, the body of Christ:

Eph 2:15-16
Having abolished in his flesh the enmity,
even the law of commandments contained in
ordinances; for to make in himself of twain
one new man, so making peace; And that he
might reconcile both unto God in ONE body...

The Bride of Christ includes OT Israel:

Rev 21:9b-12
Come hither, I will shew thee the bride, the
Lamb's wife. And he... shewed me that great
city, the holy Jerusalem... and at the gates
twelve angels, and names written thereon,
which are the names of the twelve tribes of
the children of Israel...

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The 144,000 may be a subset of a greater number
sealed:

Rev 7:3-4
Hurt not the earth, neither the sea, nor the
trees, till we have sealed the servants of our
God in their foreheads. And I heard the number
of them which were sealed: and there were sealed
an hundred and forty and four thousand of all the
tribes of the children of Israel.

For all Christians are servants of God:

1 Cor 7:22
He that is called, being free, is Christ's servant.

Rev 1:1
The Revelation of Jesus Christ, which God gave unto
him, to shew unto his servants things which must
shortly come to pass; and he sent and signified it
by his angel unto his servant John...

And there Christians on the earth even after the
144,000 are in heaven:

Rev 14:3, 12-13
And they sung as it were a new song before the
throne, and before the four beasts, and the elders:
and no man could learn that song but the hundred and
forty and four thousand, which were redeemed from
the earth... Here is the patience of the saints:
here are they that keep the commandments of God,
and the faith of Jesus. And I heard a voice from
heaven saying unto me, Write, Blessed are the dead
which die in the Lord from henceforth: Yea, saith
the Spirit, that they may rest from their labours;
and their works do follow them.

If the 144,000 aren't a subset of a great number
sealed, then God may allow Christians who are not
the 144,000 to be tormented by the locusts, just as
He has allowed men to torture Christians throughout
the centuries. They will certainly receive great reward
for their sufferings (1 Pet 4:19, Rev 2:10, Heb 11:35).