"Let him trust in the name of the Lord, and rely upon his God."

10. Who among you fears the LORD and obeys the word of his servant? Let the one who walks in the dark, who has no light, trust in the name of the LORD and rely on their God.
11. Behold, all you that kindle a fire, that encircle yourselves with sparks: walk in the light of your fire, and in the sparks that you have kindled. This shall you have of my hand; you shall lie down in sorrow.
Isaiah 50:10-11  

Matthew Henry was a Presbyterian minister (1662-1714) born in Wales.

His comments on this passage:

Those that truly fear God, obey the voice of Christ. A sincere servant of God may for a long time be without views of eternal happiness. What is likely to be an effectual cure in this sad case? Let him trust in the name of the Lord; and let him stay himself upon the promises of the covenant, and build his hopes on them. 
Let him trust in Christ, trust in that name of his, The Lord our Righteousness; stay himself upon God as his God, in and through a Mediator.
Presuming sinners are warned not to trust in themselves. Their own merit and sufficiency are light and heat to them. Creature-comforts are as sparks, short-lived, and soon gone; yet the children of this world, while they  last, seek to warm themselves by them, and walk with pride and pleasure in the light of them. Those that make the world their comfort, and their own righteousness their confidence, will certainly meet with bitterness in the end. A godly man's way may be dark, but his end shall be peace and everlasting light. 

It seems that Isaiah understands something that we sometimes forget; that following God does not necessarily mean that we should expect to feel that we are walking in light while we do so.  Sometimes following God feels like walking in darkness.  The whole chapter is written from a perspective of "this discipleship is hard" and "the Lord will not abandon me" and "this will take true grit to get through."  Which is understandable, given what we know about Isaiah and his circumstances.

The warning in verse 11 is a warning against manufacturing a substitute light (be that a belief, or a method of feeling validated, or a way of measuring, or whatever) which we may make on our own, trying to beat back the darkness and gain a sense of progress.  The light we or others create in such situations may feel like it is making our path easier to see or more positive, but Isaiah's pretty sure that living by such light will, eventually, lead to more sorrow than joy.

What is interesting to me is that, in the version in Isaiah, the warning seems to be aimed at those who do not fear the Lord and obey his servant, which might make a believer feel smug and safe. Certainly, Matthew Henry switches pronouns, using the pronouns "they" and "them" when he talks about those "presuming sinners" addressed in verse 11.

However, in the version in 2nd Nephi (chapter 7), those verses are worded so that verse 10 makes it so that verse 11 is clearly aimed at those who do fear the Lord and obey his servant. 

"Who is among you that feareth the Lord, that obeyeth his servant, that walketh in darkness and hath no light...that compass yourselves with sparks..which ye have kindled"    

It's not "them", it's "us", the ones who do fear the Lord and are trying to obey his servants, that the warning to not self-manufacture light amidst the darkness we experience is aimed at. And considering that the rest of that chapter talks about how tough and daunting discipleship can be, that makes some pretty good sense to me.