This passage is one that I most often meditate on.
The second part of the verse caught me attention in a different way:
"This is the heritage [inheritance] of the servants of the Lord."
Doesn't it seem strange that servants inherit something?
What kind of service do we render to the LORD?
"What shall I render unto the LORD for all his benefits toward me?
I will take the cup of salvation, and call upon the name of the LORD." (Psalm 116:12-13)
We are His servants we receive from Him!
The rest of Isaiah 54:17 outlines specifically what we receive from Him, which causes us to serve Him:
"Their righteousness is of me."
God does not want us to lean on our righteousness, but His.
Consider the first mention of "righteous" or "righteousness" in the Bible.
The first mention of "righteous" as an adjective appears here:
"These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God." (Genesis 6:9)
The first mention of righteousness, the noun, occurs here:
"And he believed in the LORD; and he counted it to him for righteousness." (Genesis 15:6)
Noah, which means "rest", found grace in God's eyes because He believed in Him, and received from Him. That's what made Noah righteous.
Righteousness is all about believing Him, and we receive His righteousness by faith!
In fact, since Jesus' death and resurrection, for those who believe on Him, we are made the righteousness of God in Him (2 Corinthians 5:21)
God our Father invites us to receive--and keep receiving--this gift of righteousness from Him!
"For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ." (Romans 5:17)
What makes us God's servants? When we receive His gift of righteousness! It's a hard labor for many, since so many Christians, even after they get saved and begin walking with the Lord, or determined to try to earn God's blessings, hoping that if they continue to behave and do the right things, then God will bless them.
But no! That is all wrong! We are called to receive, and to keep receiving, His gift of righteousness!