What does the word church mean?

God has always required His people to be a separate, distinct people.
From the beginning unto now, in every dispensation, God has mandated:
Separation - Exclusivity - and Holiness. The very first separation God ever
made was to separate the light from the darkness (Genesis 1:4); and in
regards to His people, He remains consistent to this day calling His people
out of darkness into His marvelous light (1Peter 2:9).
When sin first engulfed this world, God baptized it in water and
washed away its sin, separating Noah and his family from all other people on
earth (cf. 1Peter 3:21).
Another separation is seen in the Patriarchal dispensation when
God called Abraham (Genesis 12:1-3). God gave Abraham a stern requirement:
"Get thee out of thy country and away from thy kindred, and thy father's
house." That is, before God would fulfill the blessings He had promised to
Abraham, the condition of separation had to be fulfilled.
Consider when God called the Hebrews out of Egypt. Why did God
want the Israelites to separate themselves from the Egyptians? "Let My
people go that they may serve Me" (Exodus 8:20). Whether we understand it
or not, one thing becomes clear: separation was essential to their being
able to acceptably worship and serve God. That's the way it was, and that's
the way it is, because "our God changeth not" (Malachi 3:6).
In calling the Hebrews out of Egypt, God formed a unique,
peculiar people to be His very own, and gave them certain conditions of
"separation" and "holiness" to be fulfilled (Ex. 19:4-6; 1Kings 8:53), which
had to be maintained in order for them to remain God's people (1Kings
9:6-9). However, the people were rebellious toward God's word and began to
change His government over them; they began to mingle with the religions
around them. Therefore, it wasn't long before the "distinctiveness" of God's
people totally disappeared; that is, you couldn't tell God's people from all
the other people of the world! They lived, they talked, they worshipped,
they mixed with, and acted like all the other nations around them. God's
call for separation and distinctiveness had lost its edge and meant nothing
to them. God kept His promise, and eventually both the northern and southern
kingdoms were gone -- except for a faithful few (cf. Matt. 7:14) through
whom God brought His Christ into the world.
Today, we have His church -- His people. Those who respond to
the Gospel call are called out of darkness and into His Light. The very
definition of the word translated "church" or "assembly," means "to call
out." In 1Peter 2:9, it speaks of God's "royal priesthood" -- note the word
is singular, not plural, God has only ONE priesthood. The same verse speaks
of God's "holy nation" and "unique people," and note again that both words
are singular, that is, God has only ONE holy nation, His church (not
churches). His one nation operates under one Law - the Gospel. No religious
organization that operates outside His Law can be said to be His "holy
nation."
There is coming a time when a great and final separation will be
made: those who obey His Gospel will be separated from everyone else
(2Thessalonians 1:7-8). That's why God urges, "Come out from among them and
be ye separate" (2Corinthians 6:14-17).

--Toby Miller