“Therefore, my beloved brethren, be steadfast, immovable, always abounding in the work of the Lord, knowing that your labor is not in vain in the Lord.”  
1 Corinthians 15:58

The ‘therefore’ here looks back to what the Apostle Paul is telling them concerning our victory over death.  Since that is the case they (we, too) are to be ‘steadfast, immovable, always abounding in the work of the Lord’.
(Read Chapter 15 for Paul’s message to the Corinthians on Christ’s resurrection.)

Believers are to be steadfast (fixed, firm, settled, immovable).  Other translations state this portion of our verse as:
“…be strong in purpose…” [The New Testament in Basic English]
“…hold your ground…” [The New Testament: A New Translation  (James Moffatt)
“…continue to be firm, incapable of being moved…” [The New Testament: A Translation in the Language of the People (Charles Williams)]

Paul tells the Galatians who were in danger of submitting to false teaching:
“Stand fast therefore in the liberty by which Christ has made you free, and do not be entangled again with a yoke of bondage.”  Galatians 5:1

The Apostle Peter also gives a warning in 2 Peter 3:17,
“You therefore, beloved, knowing this beforehand, be on your guard lest, being carried away by error of unprincipled men, you fall from your own steadfastness.”

Standing firm, we are to abound in the work of the Lord.
Abound (Perisseuo): to super abound, be in excess, excel [Strong’s Exhaustive Concordance of the Bible]
“So also you, since you are zealous of spiritual gifts, seek to abound for the edification of the church.”  1 Corinthians 14:12

Scripture has much to say of the work and to the workers in God’s kingdom.  I will only mention a few and hope that they will be a start for our own personal study.

Concerning various forms to which men are called:
*Work with God;
“‘But now take courage, Zerubbabel,’ declares the LORD, ‘take courage you also Joshua, son of Jehozadak, the high priest, and all you people of  the land take courage,’  declares the LORD,’ and work; for I am with you,’ says the LORD of hosts.”  Haggai 2:4

*Individual tasks are given as in Mark 13:34,
“It is like a man, going to a far country, who left his house and gave authority to his servants, and to each his work, and commanded the doorkeeper to watch.”

*Our opening verse is a call to abundant labor.  
Read again 1 Corinthian 15:58

*In the works of evangelizing the world:
Paul is speaking to Timothy here,
“But you be sober in all things, endure hardship, do the work of an evangelist, fulfill your ministry.”  2 Timothy 4:5

Do we fit into that admonition?  Yes. True Christ gave some as evangelists, but our lives are to be an expression and a testimony of Christ Jesus.
Did not Jesus say in Matthew 5:15-16,
“You are the light of the world.  A city set on a hill cannot be hidden.  
“Nor do men light a lamp, and put it under a peck measure, but on the lamp stand; and it gives light to all who are in the house.
“Let your light shine before men, in such a way that they may see your good works, and glorify your Father who is in heaven.”

As we are more and more conformed to the image of Christ, let us pray for the opportunity to share the Good News, the Gospel of Christ, with those in our sphere of influence.  Always keeping in mind 1Peter 3:15,
“But sanctify Christ as Lord in your hearts, always being ready to make a defense to everyone who asks you to give an account for the hope that is in you, yet with gentleness and reverence.”

As in 2 Peter 3:9, we too desire that none should perish:
“The Lord is not slow about His promise…not wishing for any to perish but for all to come to repentance.:”

God bless each one!  Amen