Who is Hebrews 10:26 speaking to?

"We" and "us" generally stand for the writer and the people he is writing to. Let's identify who that is through some verses.

Hebrews 6:1-3
1 Therefore leaving the principles of the doctrine of Christ, let us go on unto maturity; not laying again the foundation of repentance from dead works, and of faith toward God, 2O the doctrine of baptisms, and of laying on of hands, and of resurrection of the dead, and of eternal judgment. 3 And this will we do, if God permits.
(Not unbelievers in any case.)

Hebrews 6:9
But, beloved, we are persuaded better things of you, and things that accompany salvation, though we thus speak.

(This is in direct contrast to what was said just before in Hebrews 6:4-8 about people who were saved and then fell away. Here "we" is just the writer, it's a polite way to say "I". And "you" are the "beloved," the writer's brothers and sisters in Christ that he is writing to.)

Hebrews 10:21-25
21 And having a high priest over the house of God; 22 Let us draw near with a true heart in full assurance of faith, having our hearts sprinkled from an evil conscience, and our bodies washed with pure water. 23 Let us hold fast the profession of our faith without wavering; (for He is faithful that promised;) 24 And let us consider one another to provoke unto love and to good works: 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.

(v 21: Jesus isn't the high priest of unbelievers, Jews or not.
v 22: Unbelievers don't have any "assurance of faith" to approach God with; their hearts aren't pure, their sins aren't washed away by Jesus' blood.
v 23: No profession of faith applies to unbelievers.
v 24-25: Loving one another and assembling together is something believers are called to do; encouragement to fellowship together only makes sense for people of faith, people who know Jesus/have Him as their Lord and Savior.)

Hebrews 10:26-31
26 For if we sin willfully after we have received the knowledge of the truth, there remains no more sacrifice for sins, 27 But a certain fearful expectation of judgment and fiery indignation, which shall devour the adversaries. 28 He that despised Moses' law died without mercy under two or three witnesses: 29 Of how much worse punishment, suppose you, shall he be thought worthy, who has trodden under foot the Son of God, and has counted the blood of the covenant, with which he was sanctified, an unholy thing, and has done insult unto the Spirit of grace? 30 For we know Him that has said, Vengeance belongs unto me, I will recompense, says the Lord. And again, The Lord shall judge his people. 31 It is a fearful thing to fall into the hands of the living God.

(The writer is still addressing the same people he had been addressing in the verses before verse 26. They are the body of Christ, the people of God, not unbelievers.
v 30: The reminder that "The Lord shall judge his people" is for them precisely.)

 

"I will never leave you nor forsake you" (Joshua 1:5).

That word was spoken to the people of Israel who had been obedient to God's voice and were entering the promised land. The people who had been rebellious and sinning died in the desert; they didn't make it into the promised land.
The apostle Jude thus reminds in verse 5 of his letter:
"Now I want to remind you, even though you are fully aware of these things, that the Lord who once saved his people from the land of Egypt later destroyed those who did not believe."*

So, that word in Joshua and that also appears in Deuteronomy was not spoken as a promise to people who were being willfully disobedient to God or in a backslidden state. We shouldn't suppose that word applies to anyone who is a backslider who has not made up his/her mind to come clean with God. Such person should urgently repent or will perish (e.g. Revelation 2:5, 2:16, 2:22, 3:3, 3:19 - all these are warnings to backslidden people who were once saved). People do fall away from the faith (from being previously in it (Hebrews 10:38-39, 2 Peter 2:20-22).

Jesus spoke similar words in Matthew 28:20:
"I am with you always, even to the end of the age."

As in the word in Joshua, attention should always be paid to who is being spoken to. Jesus was speaking to his disciples who were on fire for Him and not messing with sin in any way. But it is a fact that people today keep wanting to apply such a promise to themselves, even as they are lukewarm and are still deliberately sinning. A relationship with God implies the faithful obedience of his servants out of love (1 John 5:3), and that's opposite to going on sinning. It's important to point that out as false teachings make up most of what "Christian" ministers preach (1 John 4:1 warning).

 

See also:
Willful Sins and Unintentional Sins
Sinning Willfully and Sinning Repetitively
Comments on Willful Sin After Salvation and Eternal Security (Focus on the Family)
*Faith Implies Obedience
Can You Lose Your Salvation?