Q: How do you understand Hebrews 10:25?
Hebrews 10:25 Not forsaking the assembling of ourselves together, as the manner
of some [is]; but exhorting [one another]: and so much the more, as you see the
day approaching.

A: Notice that Christ said in Matthew 18:20 that He would be in the
midst of two or more of those gathered in His name.

Matthew 18:20 For where two or three are gathered together in my name, there
am I in the midst of them.

Therefore, two people could well qualify to fulfill Hebrews 10:25, as far
as number considerations.

Moreover, one must not assume that a large church assembly is Paul’s
intentions in Hebrews 10:25.  Notice that the reason for getting
together was for the purpose of exhortation or discussion about the
culture of those present and whether it squares with the God culture, as
we will see when we discuss verse 26.

To exhort means to use strong, stirring argument, admonition, advice or
appeal.  This would include urging a brother to hold to the keeping of
God's Ten Commandments and sticking close to God.  It would include
the proper understanding of the Covenant of God - what His part is and
what our part is.

Definition of Exhort: To urge by strong, often stirring argument,
admonition, advice, or appeal: exhorted the troops to hold the line.

Such exhortation in today's organizations would probably get one
thrown out of that group, as often a doctrine held by a given
organization does not square with the bible; and I personally know of no
one who would endure strong exhortation in this time in which we live.  
Rather, most select to go to hear preachers or teachers who will say
soothing things or will not require a great deal from them.  
Furthermore, the preachers love to have it so, as they can repeat the
same doctrines over and over that they know the people will accept and
the money keeps rolling in for their exorbitant living.

2 Timothy 4:3 For the time will come when they will not endure sound doctrine;
but after their own lusts shall they heap to themselves teachers, having itching
ears;

In addition, we have the example of Abraham the father of the faithful.  
Abraham and Sarah lived on a mountain in Canaan and eventually had
to separate even from Lot in order to keep the peace.

Galatians 3:7 Know ye therefore that they which are of faith, the same are the
children of Abraham.

Notice why Hebrews 10:25 is important:

Hebrews 10:26 For if we sin willfully after that we have received the knowledge
of the truth, there remaineth no more sacrifice for sins,

Therefore, we must conclude that Paul was aiming at those who were
going back into the world - returning to a life lived by the carnal mind
and one that would result in sinning against the holy spirit thereby
losing their salvation.  

Each person must determine for himself if his attitude is tending toward
life or death.  Recall again, what the coming together was for: to exhort
one another, not necessarily to hear a sermon - although a sermon can
be one of exhortation.  If the person giving the sermon has God's holy
spirit, it is very possible that he will touch on course-correction items of
importance to several in the group.  

On the other hand, if just a few get together - two or more - Christ has
promised to be in the midst.  Moreover, if those who get together use
the time for the purpose of sharpening their understanding of what God
requires of His people, then it should fulfill Paul's requirements in
Hebrews 10:25.  Today, I find this takes place by osmosis (A gradual,
often unconscious process of assimilation or absorption) rather than
anything like direct exhortation.

If one sleeps through a dull sermon and talks about how tired he is after
the sermon or talks about mundane (worldly, secular) things, one can
see how a group could assemble together and not even come close to
accomplishing what Paul intended in Hebrews 10:25.  One should not
over-emphasize the assembling and overlook why he is to assemble!  

It could have been in Paul's day that times were so dangerous for God's
people to get together that they were beginning to drift back into the
world and forget their covenant with God.  Notice what Paul said in
Hebrews 2:1.  

Hebrews 2:1  Therefore we ought to give the more earnest heed to the things,
which we have heard, lest at any time we should let [them] slip.

The implication is that many were losing their first love and slipping
back into Satan's society.  Once a person has received the holy spirit,
this lifetime is his only time of salvation.  Paul seemed concerned that
many of the Hebrews were losing their only chance for salvation and he
made an argument in the book of Hebrews for them to use every means
at their disposal to make sure they did not lose their eternal life.

Assembling with like-minded brothers can be positive.  If all are pulling
together, life can be easier than when one must pull alone:

Ecclesiastes 4:12 And if one prevail against him, two shall withstand him; and a
threefold cord is not quickly broken.

However, if the time going to and from the meeting is greater than the
time of the meeting itself, one must question such effort.  On the other
hand, if one finds that such meetings draw him closer to God then he
should take advantage of such gatherings.  These are difficult times.  
Often assemblies are far away.  Each of us must do what is necessary
to put the Kingdom of God first in our life and work out our own
salvation with fear and trembling.  The result is what is important; and
if one has the holy spirit, he should be able to gage whether he is
making progress toward the kingdom or to the contrary.

Philippians 2:12  Wherefore, my beloved, as ye have always obeyed, not as in my
presence only, but now much more in my absence, work out your own salvation
with fear and trembling.

Peter put it this way:

2Peter 1:5  And beside this, giving all diligence, add to your faith virtue; and to
virtue knowledge;
6 And to knowledge temperance; and to temperance patience; and to patience
godliness;
7 And to godliness brotherly kindness; and to brotherly kindness charity.
8 For if these things be in you, and abound, they make you that ye shall neither
be barren nor unfruitful in the knowledge of our Lord Jesus Christ.
9 But he that lacketh these things is blind, and cannot see afar off, and hath
forgotten that he was purged from his old sins.
10 Wherefore the rather, brethren, give diligence to make your calling and
election sure: for if ye do these things, ye shall never fall:

One must be careful not to make the Sabbath Day a party.  The
Sabbath is to meet with God, not to meet with family and friends in
public restaurants.  However, most cannot endure strong exhortation:
An example of what Hebrews 10:25 is all about.  In other words, the
assembling part is easy, it is the strong exhortation - the very reason
for getting together - that is hard to accomplish and most often missing;
and what one will most often not endure.

RSV:
Isaiah 58:13  "If you turn back your foot from the Sabbath, from doing your
pleasure on my holy day, and call the Sabbath a delight and the holy day of the
LORD honorable; if you honor it, not going your own ways, or seeking your own
pleasure, or talking idly;
14 then you shall take delight in the LORD, and I will make you ride upon the
heights of the earth; I will feed you with the heritage of Jacob your father, for the
mouth of the LORD has spoken."

Questions ...
Questions & Answers
HEBREWS 10:25